Someone once said that there are ten thousand books published every day. I doubt that number is accurate, but I imagine in our day, a day where internet and self publishing are more easily accomplished than in times past, the amount of people writing (and "publishing") each day far exceeds that number. If you attempt to keep abreast of what everyone says, it will soon be seen as a futile pursuit. One cannot possibly read that much material, nor can many afford to do so financially.
So I'm going to give you the best advice I can concerning how to build your library. This is advice I wish I had when I first started my library twenty years ago. I would have wasted less time and money buying things that were of little to no value in what I wanted to accomplish with it. So here is my list of do's and don'ts of buying non-fiction (mainly academic) books in order to build your research library (in other words, I'm not talking about devotional or books on challenging subjects or controversial issues, but books you would use for sources of academic knowledge on a subject). All of the following are only general guidelines, and are not always the case:
1. Don't buy every book that comes out because its in a subject area that you like. The book may be complete trash. Instead, look at the publisher of the book, the qualifications of the one writing it, and whether the endorsements come from reputable sources.
2. Don't buy every book on the subject even if it is written by reputable sources and published by top publishers, but instead, buy the best two resources on opposite sides of the debate. If you wish to dive further into the subject, it can't hurt starting with only the two best books that represent both sides of the debate and go from there.
3. Don't buy a bunch of books that are merely summations of more academic books. In other words, twenty books may be getting their information from one or two academic books that make the argument. If you buy these twenty books, it's not only redundant, but it also will likely not allow you to see the argument laid out as well as you should if you are to critique it or assimilate it into your thinking.
4. Wait for books to go on sale, buy used, and try to get them on ebay as live auctions rather than those auctions with a Buy It Now price. Most of my library that is now worth around $25,000 used, would have cost me around $70,000 had I bought these books brand new. Don't waste thousands of dollars on books because you like the smell of new books. If you absolutely need that smell, buy one new book a month and smell it every time your reading a used one.
5. Don't overwhelm yourself with a massive buying streak of books. Simply buy two, each from opposing sides, and sit down and go through them one by one. If you buy tons of books at a time, you'll never end up reading them, wasting your money (which is bad stewardship), and only give yourself a delusional sense of being informed because you own a book on the subject.
6. Always read the reviews on Amazon and elsewhere on the internet to get some sense of what the book is arguing and whether or not it deals with what you want to really study. I've bought countless books before thinking they were going to address a particular subject, only to find that the book didn't even touch it.
7. Consider using a nearby library, if it is a good one, rather than buying books that you will not use again and again. If the local library has the books you need, and you'll only read it once, why buy it other than for the sake of the vanity of having it on your bookshelf. Save your money for buying books that you will use more than once.
8. Take notes when you read, so that you don't have to either buy or hold on to a book. Chances are, if you take good notes, including page numbers, you won't really access the book again.
9. Trade or sell your read books so that you'll have credit or money to buy other books that come out. This will give yourself an incentive to read through a book and take good notes, knowing that you will not always have it and now desire to get the next exciting book that has come out.
10. If you have plenty of money, give all of your books to me. Just kidding. I couldn't think of a tenth piece of advice, so this seemed like the logical thing to do, and perhaps that is my advice: beg for books when you can't always afford them, ;-) Obviously, however, if you do form a group of people with whom you can share, or even go through, a book, you will be more likely capable to afford and go through a book with greater efficiency and in less time.
The summary of all of these suggestions is that instead of seeking to grow the size of your academic library, set your mind to shrink it or keep it at a manageable size. This way your library will be in your head, where it should be, and not taking up too much space in your house. You'll be wealthier and more knowledgeable for it. And isn't that the point of an academic library anyway?