There is no right or wrong way to buy books if the aim is the simple pleasure of reading. And since fewer than half of all Americans read a book once they graduate from high school, God bless that exceptional person for being a reader. But collecting books versus just buying and reading books is an activity that requires organization, purpose, and planning. Starting questions that collectors must ask are, "What am I collecting, and for what reasons?"
A few people indiscriminately collect "old" books as investments because they intend to resell them some day. That can be a disappointing strategy if profit is the goal because the age of a book often has very little to do with its value. Book dealers, collectors, and librarians, however, do use some broad time spans to establish dates of books with likely importance and value: e.g., all books printed before 1501, English books printed before 1641, books printed in the Americas before 1801 and books printed west of the Mississippi before 1850. Yet, even these dates are rough guidelines at best and are always subject to the overriding factors of intrinsic importance, condition, and demand.
"Intrinsic" importance really has to do with what is important to the collector himself. For example, I collect books written by Larry McMurtry. I suppose I have several copies of everything he has written, yet only a first edition, first printing Lonesome Dove--with a specific (and single) typographical error--is really worth much, and then only about $100. Still, there is something about McMurtry's style that I find truthful, lyrical, and elegant in a laconic way. I collect him because I like the writing.
Another collector is the "accidental" collector who begins reading someone like Sue Grafton and her "alphabet" series of mysteries. One day the reader notices that she has "A" is for Arson and "C" is for Crime but is missing "B" is for Burglar. Suddenly the reader has a mission. And, since Grafton is up to "S" is for Silence, we can only assume that our accidental collector will one day own all 24 Grafton Titles.
Some books are always in demand by collectors. These include early editions of novels by the trinity of American literature, Faulkner, Fitzgerald, and Hemingway (Faulkgeraldway in book-speak). There are also books that represent a transition point in literature such as Ulysses by James Joyce, Catcher in the Rye, by J.D. Salinger, or On the Road by Jack Kerouac. Certain books by these writers can be worth as much as two or three thousand dollars--or more.
Other collectors are people who are only secondarily interested in books, but who are interested in a particular subject such as the Civil War, certain makes of cars, or birdhouses, and on and on. No matter how esoteric or narrow an interest may be, a writer--and maybe several hundred writers--have written books about it. "Of the making of books there is no end" and thank goodness, for otherwise there would be no occupation for booksellers like me, or for librarians, writers, and publishers.
A lot of young people (and some not so young) have started collecting Harry Potter books. While I can't argue the literary merits of Rowling's oeuvre--I wasn't able to finish the first of her novels--I am quite certain that first editions/first printings of her books, especially UK editions, are going to be worth some serious money. I am always happy when I find one at garage sales, or in a jumble shop somewhere.
Conversely, Stephanie Meyer's vampire books, among them Twilight, as an example, will never be worth much, if only because the publisher printed about a zillion first editions/first printings on relatively cheap paper. The abject silliness of a book rarely enters into a bookseller's assessment of its future valuation since lots of profoundly goofy books are highly collectible. In Meyer's case, however, production factors plus stupid equal ho hum.
Books written by people who have never actually read a book--so called public intellectuals like Al Franken and Michael Moore on the political left and Glenn Beck and Rush Limbaugh on the right--aren't worth any money at all less than 30 days after publication, and are functional doorstops by day 31. These "writers" are never collectible in the way that genuine public intellectuals, such as Ambrose Bierce, William F. Buckley, G.K. Chesterton, H.L. Mencken, and Mr. Dooley will always be.
Good bookstores are characterized by the number of informal collections it has amassed, and which are interspersed among the general run of books. Because I love Graham Greene, Stanley Elkin, Harry Crews, and Hillarie Belloc, to name just a few, I always have several of their books on the shelves--and they stay there because these writers are simply out of fashion. Even though they probably will never sell, I can't resist buying even more copies. If you find yourself in the same fix it is safe to say that you are a collector.
If you are interested in collecting books you should stop in and have a conversation with an independent bookseller. She or he will be very glad to help you get started on your collection--especially if you want to start in their shop!