I've had a lot of books in my lifetime, but unfortunately I've moved house many, many times and had to shed books along the way. Some have remained with ex-partners, and new partners have then filled up my shelves with their books, and so on…it's a kind of meiotic process that lead to strange new genetic combinations! Sadly a lot of our books and bookshelves aren't easily accessible at the moment - my husband works from home as a non-fiction sales rep, so a good many of our own books have had to be packed away to make room for his stock, whilst others are on bookshelves wedged behind furniture - or, in one case, a bicycle! - which makes for terrible photos.
So, I've concentrated on the bookcases that are important to me as a writer. The first, of course, is my SF and fantasy collection, in the corner of the bedroom. This holds everything except the complete Terry Pratchett collection, which practically needs its own bookcase! It's a mixture of old favourites and new, and a few TBR titles on the top shelf - I mostly buy ebooks these days, purely to save space, though if I really love a book I may buy the dead trees edition as well.
Because it's arranged alphabetically (apart from the TBR section at the very top), you can't quite see the battered paperback three-volume copy of The Lord of the Rings, which is probably the oldest book on there. You _can_ see the 1970s edition of the Deryni Chronicles by Katherine Kurtz, which I bought when I was an undergraduate, and my fairly comprehensive collection of Lynn Flewelling paperbacks (with a space kept for the final Nightrunner book, due out next year I believe). And yes, those are Argonath bookends from the Lord of the Rings DVD boxed set - sadly my cat has chewed their outstretched hands, so they're not exactly collectors' pieces any more!
Second is my main reference/research bookcase. This holds a bunch of writing books - again, not all I've ever bought, but the ones I currently feel are worth holding on to - and most of my historical research. The second shelf down is almost entirely Elizabethan stuff, from general social history, through specialist topics like the Elizabethan secret service, to biographies of famous individuals such as Sir Francis Walsingham and Sir Walter Raleigh. My favourite is probably "The A-Z of Elizabethan London", which is an annotated reprint of a map c.1570 - it's been invaluable in writing my historical fantasy series "Night's Masque". The layout of the main streets has changed so little that one can still use it to navigate from the reconstructed Globe on Bankside to the Tower of London (something I did once, to see how long it would take my characters to walk the route).
The lower shelves are a more mixed bag: ancient, medieval and early modern history; books about Venice (the middle book of my trilogy is mostly set there) and other Italian cities; the history of technology; and for some reason several books about sex in history! Well, perhaps not so surprising - it's one of those topics that tend to get skirted around in school history lessons, but as a writer it's as important to know what your characters' attitudes are to sex, as it is to know about dates of battles or the economic effects of the Black Death.
The third and final photo shows the bookcase that sits next to my desk, within easy reach when I'm at my computer. On the lower shelves are more reference books, this time with the emphasis on language: dictionaries, and books about linguistics. I've been interested in constructed languages since my teens (even before I read Tolkien), and they constitute a small but vital element in my books. Naturally I also need to know about the history of English and how it was used in Shakespeare's day, and then there are the random foreign and specialist dictionaries I've accumulated over the years.
The upper shelves are a mixture of business and pleasure. This is where I keep both my own "archive" copies of my novels and (on the shelf below) the spare author copies I'm saving for giveaways (not many left now!). As well as my books, you can see there are art materials - drawing pads and boxes of pens - and a box of Playmobil people (to go with the Playmobil pirate ships on my desk); a 6" figure of Spike from "Buffy the Vampire Slayer"; and the obligatory writer's bottle of whisky (in my case, Glenkinchie 12-year-old single malt). Hanging from the righthand side of the shelves is my collection of convention badges, most prominently the one from this year's WorldCon. Hiding away in my study writing novels can be lonely, so I love to get out and meet other writers - and of course readers!