A good library contains resources which enable you to have a more clear understanding of what the Bible says. Many different types of tools exist to provide information to help in this pursuit. (There are also books that discuss practical aspects of Christian living. Though these are useful in their own right, these are not in view in this discussion.)
There are several different types of books that together make a useful library. These are:
Since books are expensive, it is important to make sure you get maximum utility out of the books you purchase. Here I will discuss each of these types of books, with some suggestions for how to best spend your money.
This is the most important type of book in your library. You may want to include several different translations of the Bible. The New International Version is very readable and accurate. The New American Standard Bible follows the structure of the original languages more closely, but is slightly less readable. It is an excellent translation.
Paraphrased bibles like the Living Bible should be avoided because they are highly interpretive. If a simpler English version of the bible is desired, the bible translation for the deaf is useful. This version is translated into words that are more easily expressed in sign language.
The King James version ought to be avoided because the English language it was written in more than 300 years ago had changed quite a bit. A translation ought to be in the vernacular of the person reading it.
Use much care in purchasing study bibles. Many of these contain excessive notes that represent themselves to "explain" what the Scriptures mean (a one-sided commentary). These notes have a way of directing one's attention away from the Scriptures themselves. These type of study bibles should be avoided; some examples are the Ryrie and Schofield study bibles. Other study bibles contain useful references, maps, small concordances and other information without one-sided commentary on the scriptures. Among these are the NIV Study Bible and the Thompson Chain reference bible. These can be helpful.
A commentary explains details about the setting of the individual books, and systematically discusses individual passages of scripture. Look for multiple points of view to be considered and explained, with the merits of each treated fairly. You are NOT looking for a commentary that will explain how every passage ought to be seen. Rather, you are looking for a commentary that will explain how every passage has been seen, or could be seen, and why it has been seen that way.
The most important thing to look for is an even-handed treatment of controversial topics. It is OK if a commentator has an defensible opinion about a specific point of view; but you are looking for a detailed and fair treatment of each point of view. Then you can use this information to come to your own opinion.
A good commentary comes in a set of many volumes and can be quite expensive. Use extreme care in purchasing a set of these. Another factor to consider is the level of expertise. Some are general, some provide extensive discussion of the original languages and other more detailed issues. So, pick a commentary that is at your level of knowledge, but that is also fair and not one-sided.
Concordances show the location of all instances of a particular word in a given translation. These are of limited value, since numerous subtleties exist in translation process. For instance, in a particular passage the English word "love" may have been translated from any number of words in the original language, or it may not have been in the original language at all, simply being added by the translators to made a sentence more understandable in English. Again, the word "love" never appears in the book of Acts, yet instances of love are found throughout. Such are some of the limitations of word studies.
Small concordances are found in many bibles and are mostly useful for finding passages based upon a key word. Larger concordances have their limitations (see above) and should only be used with those limitations in mind.
Exhaustive word studies ought to be done using the original Bible languages, and the language tools available are an excellent resource in this area.
Many different types of language study tools are available, but great care must be exercised here. Biblical language expertise only comes by extensive study, and many tools purport to provide a casual student with this expertise. This is like saying you can learn to fly a plane after a two-hour seminar! These tools are misleading; among them are Vine's dictionary, interlinear Greek bibles, exhaustive concordances, do-it-yourself Greek study books, and the like.
For the beginner, the abridged "Little Kittel" Theological Dictionary of the New Testament by Bromily is a great value. So is the four-volume Dictionary of New Testament Theology by Colin Brown. These contain fairly elaborate discussions of key Bible words and how they are used in key passages.
Another useful tool can be a lexicon. This tool provides precise definitions of words in the ancient languages but lacks the elaborate discussions of the theological dictionaries. The Arndt and Gingrich translation of Walter Bauer's work is the standard Greek lexicon. A shorter version of this lexicon, containing only words from the New Testamemt, has been produced by Danker.
If you are really interested in language studies, it would be prudent to take a course in New Testament Greek at a local seminary or university. You will then get usefulness out of good tools like the Greek New Testament and those mentioned above, as well as commentaries and more specialized works that consider the original languages of the Scriptures.
Good tools in this category tend to be expensive. Don't waste your money here with poor tools or tools that you don't have the expertise to use.
Most bibles contain a general set of maps that are useful for general purposes. Some specialized atlases are available. What you are looking for in this category are atlases that discuss locations relevant in different biblical ages. A map of Israel during the time of the Judges is different than one during the New Testament era. A good atlas in this category can be a great and useful investment.
Many bible encyclopedias are available. Some are expensive, some are not. You generally tend to get what you pay for in this category. Encyclopedias attempt to cover the ground of atlases, commentaries and other specialized works. Cheap encyclopedias tend to be like one-sided commentaries, but good encyclopedias are a useful complement to the more specialized commentaries and other works available, and are a great investment.
A good survey is an excellent investment. It should contain articles on each of the books of the New Testament or Old Testament, with some specialized articles as well. It is a great place to start a library, and a good investment to have prior to purchasing a set of commentaries.
Many other books and products available can be a great addition to your library, but be careful to consider the same cautions on products mentioned above.
The primary keys to your investments here are your study interests and budget. If you want to study everything and can afford everything, then buy everything. If you're like the rest of us, use care in purchasing. Purchase quality rather than quantity. I would rather have one good commentary than ten bad ones. Poor quality books do more harm than good and (justly!) end up in boxes stashed away.
For most people, the place to start is with several good translations of the bible (possibly a study bible) and a good New Testament survey book. Then add an encyclopedia or a good set of commentaries. After this, you will know what areas of expertise you want to grow in. This will point the way for what additional tools you ought to purchase.
I would suggest that great care be used in purchasing any books or software of a spiritual nature. You can waste a lot of money in a "impulse buying" items that will not get you what you thought they were going to get you. A good rule here is to buy works by authors or publishers you know, or those recommended by someone you trust. If you bravely purchase a book or product without knowledge of the author or trusted recommendations, you simply never know what you are going to get.
One more tip: Look for books and software on sale! Most bible and seminary bookstores (and on-line bookstores) have sales. Find out when they are. It is quite common to get products for 50% or more off list price; which can be a great help on more expensive items.
And purchase from sellers who share your values. That means go to a place like Christianbook.com. Stay away from secular sellers who do not support Christian values-- why give them your business over somebody who shares your values?