I have found a positively dangerous site on the Internet....BetterWorkBooks.com. It is a US site which sells used books for the benefit of literacy projects worldwide. Prices are very reasonable and the postage is also reasonable with the added advantage of it being carbon conscious.
I have to restrain myself but when our dollar was up against the US dollar I managed to fill some gaps in my collection of children's books and add a couple of dictionaries to the work collection.
Our local Vinnie's has a very good book section, a large room at the back. It is well organised. The fiction is in alphabetical order - paperbacks separated from hardcover. (There is very little of the latter.) The non-fiction is in groups - usual masses of cooking and gardening, some craft, crosswords and health, travel etc. Prices there tend to be reasonable too - especially if you happen to hit a half price sale day.
The problem is that Dad and I like to collect books but we do not like to pass them on. We almost certainly have too many books (although we like to think that this is not possible). I know there are still books in the shed - and we already have the shelves in here double-stacked.