I am a book dealer on the Internet. I started this business as a way to get rid of the hundreds of books I had accumulated. Now, years later,I have thousands of books piled in the attic.
Oh, well. I caught the bug. And I know what it's like to buy a $4.00 book on eBay (on Syphilology) - and sell it the next week for $95.00. I know what it's like to start with nothing and to have $120,000 worth of listings in two years, working odd hours.
Here are some tips for collecting books that will become valuable. I can guarantee that these suggestions will work well over the long haul.
First, don't fool with books unless you love them. Their mysteries are endless, their variety limitless - but I can't imagine taking this enterprise on unless books really captivate you - and unless you've been something of a book lover- a bibliophile - over the course of your life.
Second, do your homework. If you see a copy of an art book in the local thrift shop, and you think it might be very valuable, and they want 8 bucks for it, make a note of the title, author, and publication date, and check it out on Amazon. You'll know within seconds whether it's worth $6.00 or $60. A few book dealers use their cell phones to do this on site. It's called "scouting," and it amounts to a legal form of cheating. The good news is that it's too much effort for most people.
Third, value is derived from high demand and low supply, so don't purchase books that have printed in large numbers. In fact, avoid all books published in the last twenty years, even if they're signed by the author and Princess Margaret, too. There isn't a Tom Clancy novel or John Grisham that worth paying more than pennies for, and that's if it's mint. One exception to recent books is if the book was printed in limited quantities, perhaps for an academic audience. Keep an eye out for names of obscure publishing houses.
Fourth, work from strength. If there are areas of expertise you have developed, take advantage of them. If you know a lot about golf or archery or ceramics, then take the time to find out what's out there. Buy old books in your area of expertise and examine them. The good ones are bound to be worth a lot some day. The bad ones? Well, you learn from them, too, and someone is bound to want them.
Fifth, all old books that have something to say and look good will appreciate in value. Period. And the reason is that publishers can't afford to print books profitably any more - so all the books in print are irreplaceable. What that means to me, for example, is that when I buy old science fiction paperbacks, with green men and spaceships and the like on the cover, that these will be worth more and more. I sell paperbacks that I bought for 50 cents for $10.00 every day of the week - if they are nice looking ones. The same is true of military books, mysteries, and biographies.
Sixth, Baby Boomers are retiring and will be reading more and more - so keep an eye out for copies of books that mean something special to them - like James Bond first editions, or books on surfing or rock music. Similarly, Vietnam books from the 70's are hot sellers.
Seventh, buy low. Set yourself a limit of a few dollars and go to thrift shops, church fairs, garage sales, and even eBay. Even if you you make a mistake, you will learn from it. There are plenty of one-dollar books that are worth a lot of money, and you will find them if you use your wits and your patience.
Eighth, use your brain. The example above, the medical book on Syphilology, is an example of my not being afraid of fancy words. Syphilology is the study of syphilis, and that book, a leather-bound beauty from the the 1890's, had many gruesome photos of disease victims. Turns out a practicing physician had been looking for this volume for years, and was pleased as punch to find a good one and pay big bucks for it. It makes sense: there was unique information in that book that will always have value. It wasn't outdated, it wasn't silly, it looked good. Don't be put off by weird academic topics. You don't have to read arcane books, you just have to own them!
Finally, you will run out of space before you run out of money when you invest in books, so be prepared to find nooks and crannies for your volumes, and make sure your spouse is on board.