I love a good trawl of boxes of book at jumble sales, second hand shops and car boot sales!
Over the last 3 years, by such means I have gotten 3 Doctor Who Books, from the early 90's til 1997 Virgin New Adventures Range for £1 each, and sold them for between £50-£65 each! Lungbarrow and Dying Days could be as rare as Hen's Teeth even not too far past their publishing date, especially Dying Days as it was the last of the range, and the only 1 to feature the 8th Doctor, and I think for various reasons it also had a far more limited print run than was normal for the range so Collectors who want the full NA Set are desperate to get missing copies in decent condition, the 3rd was RTD's VNA Novel "Damaged Goods" which of course was set mainly on a Council Estate in London, and featured a chavy London lass called Rose Tyler.... not sure how rare it is or ever was, but I suspect the value increased since RTD brought the Show back on air, seeing as in some ways it is a prototype for some elements of New Who, which has of course since its return happily borrowed from the VNA Range, with the awesome Human Nature being fully adapted into a 2 parter. The copy of Dying Days which was near mint that I managed to find went to an extremely happy chap at Uni in Ireland, Dying Days was the very last Novel he needed to complete the entire collection of both Virgin New Adventures (all 7th Doc except for Dying Days) and the Virgin Past Doctor Adventures (various novels utilising the classic era doctors)
The money raised went on more books for myself, and computer parts & games
I mostly trawl to find stuff I haven't read, but its nice to run into stuff I know or suspect is worth a bit of money if in good condition, since it assists in purchasing more. I don't really have a collectors mindset, I read books, if I really love them I sometimes keep them, if not or if space is short, as I have a very small flat I donate them to charity shops. I still apparantly have around 6 large boxes filled with books in the attic of my ex fiance's Father's House in Leeds, be nice if I can afford to get it collected one day, I can't remember whats there, but I think there were several books of potential value, thankfully my ex fiance was not into books, so likely has no clue if they are worth anything, everything else of mine that did have value she sold! didn't even share the cash, which was rude lol
My local Library usually has a big shelf full of books that are old, or in poorer condition for 25p and 50p, often get some good stuff there, like some old Starwars EU Novels. We have an Oxfam Bookshop, which got in a load of Star Wars EU Novels, including several of the awesome X Wing series, so I enjoyed re-reading some "old friends" as books can be, then donated most back, except for certain X wing Novels, as my Brother pretty much mugged me for them, well he gave cash, insisted on it! but I had no choice, they filled the big ugly gap in his X Wing collection, as he is an avid collector of certain things, and he had been trying to fill his X Wing collection for years, couldn't find them anywhere, and the only ones on places like Ebay were ridiculous prices. The X wing novels are awesome sauce, all the way! Especially the running joke about an Ewok X Wing Pilot. I forget which EU Book it was, but I do recall there being an Ewok Gangster!