Books. Have to have them. Have to hold them, open them, smell them; have to put them on shelf after shelf, sometimes double deep; have to stack them on every flat surface; have to put them in boxes, then label the boxes and stack them; have to continually load ISBN's and LCCN's into my BookCat software so I know what I have, so I won't buy the same book twice...well, not too often, unless it's a hardback of something I just have in paperback, or maybe a first edition, or an illustrated edition, or a limited edition from a small press bound in acid-free paper; or maybe the one with the Krenkle cover rather than the one painted by Frazetta. And they don't really take up too much room...I mean, as long as you can thread your way through the shelves and stacks, as long as they are structurally sound, for the most part, able to withstand all but the worst quakes. And there is always the refrigerator and oven when I start to run out of room; if the house begins to list on its foundation, I can always shift boxes around.
What I've described is a bad case of bibliomania, an extreme passion for collecting, or accumulating, books. I do not have "the gentle madness" as badly as I have described above, nor is the example cited the worse case on record. If you read some of the anecdotal tales related in books about bibliomania, you'll find the story of a man who starved to death because he had no money to buy food, while surrounded by priceless books; you'll also read about the Englishman who, when he filled a house with books, would start construction on another house. These cases are extreme, and collecting books need not develop into something resembling OCD. There are always limits to consider, not the least of which is money.
I like to think that collecting books is a way of bringing order into one's life, a way of preserving something out of the past that, if we let it slip away, civilization will suffer for lack of it. When I see a Kindle commercial, claiming "the book is reborn", I just want to throw something at the screen. A book is a book, and, believe me, a flat screen is not a book. They are the equivalent of the readers we first saw in "Star Trek," but let us not forget that some of Captain Jon Luc Piccard's most prized possessions were bound copies of Homer and the Complete Works of Shakespeare. When the crew of the Enterprise encountered Mark Twain in "Time's Arrow," he told them, quite accurately, "If you want to know who I am, read my books." Of course, these days that also means finding an edition of Mark Twain that has not been censored by some well-meaning liberal academic. When we save books from oblivion or destruction, we also rescue a writer's voice. Those who would otherwise be silenced by time and decay get a chance to live again through us; and we gain a wisdom greater than our own.
What should you collect? I like to think it unnecessary to advise that you only collect that which interests you, but there are plenty of people who get into book collecting for all the wrong reasons -- they want to become rich, they want to leave something for their children and grand children, or they just want something around that will make people think they are really smart. All these are wrong-headed reasons. You should collect what you want to read. Although it's nearly impossible to read every single book you own, you should not own a book that you would never read.
Me, I have several areas of interest in which I collect books, some fairly generalized, others extremely narrow. I collect anything about Sherlock Holmes; old paperback mysteries; books about Oman; anything about the Byzantine Empire; books about espionage, cryptography, spies and terrorism; anything relating to HP Lovecraft and the fantasy writers of his circle; books about different facets of woodworking and classic hand tools; anything about Doc Savage; Bibles and books of scriptural scholarship; westerns; pulp magazines and reprints of pulp fiction; guidebooks to London; books about Victorian times and books from Victorian times; "Man From UNCLE" books and other tie-in books to 1960's TV shows; art books; criminology and forensic sciences; and biographies; and a few others.