As you contemplate a library, I suggest you start with a bible commentary or two. These will prove helpful as you spend that devotional time in Scripture. Hearing from a couple of godly men on a passage or verse can correct errant thoughts. The challenge is that a comprehensive commentary cannot be purchased as one book. For eg. John Calvin has three volumes just to cover his thoughts on the Book of Isaiah. While Matthew Henry covers all of Scripture but it takes six volumes for him to do it. So, to begin, you will need to buy one commentary that covers only a book of the bible or a handful of books. There are one volume commentaries covering the entire bible but, because of space limitations, the amount of verses covered will be fewer.
Nevertheless, you have plenty of time to invest your dollars, to read and to study. If you rush it, you will go broke! I am thankful that my church congregation has been willing to budget money each year for me to purchase books for my personal library. However, I did not have that my first twenty years of book buying. Up until then, I would scour thrift stores and used-book stores for deals, and load up my birthday and Christmas wish lists with books I hoped people would get me.
Today, you can get nearly any book written throughout history. There are print-on-demand companies that will photocopy an old book into a new book binding. They will do this in hardcover or soft. The books don’t look as pretty but you get to read authors that were nearly lost to time. What’s more is that its a real book you can write in.
The one limiting factor on book availability is language. There are some respectable theologians who wrote in German, Dutch, Latin, etc. If these have never been translated into English, then they are out of my reach. Campegius Vitringa (1659-1722) wrote a Commentary on the Book of Isaiah that I wished I could read. It is currently available in German. Darn that Tower of Babel!