Our county's public library system has a huge sale once a year. Stuff is super cheap. Some of it is books that no one was checking out, some are in such crappy condition that the libraries just don't want them anymore, and some are over-stocks on things like Harry Potter and the Deathly Hallows.
The first morning and afternoon of the sale are for school system employees, who get to take the books for free (assuming they're for the libraries). That evening (today) and the next day are $4 per hardback and $2 per paperback. My uncle's taking the day off of work tomorrow to hit the tables. Saturday is $2 per hardback and $1 per paperback, and Sunday is $5 per box (as provided by the sale), and $2 per bag (as provided). It's just a toss-up between whether you want first pick at a slightly higher price, or all of the books no one else wanted for super-cheap.
Some highlights from my picks are a lot of Tamora Pierce novels, Margaret Zimmerman Bradley's Avalon series (although I think two of them were written by her friend after Ms. Bradley died), American Gods by Neil Gaiman, and my favorite Garth Nix book (Lirael).
I was keeping my eye out for Cory Doctorow and Terry Pratchett (the former of which I love and the latter of which I've heard loads of good things about), but had no luck.
Another series I completed my collection of is the Rhapsody quartet (although it has a real name that's not Rhapsody-- that's just the name of the 1st one) by Elizabeth Hayden. If you like fantasy and have the time/inclination to read four fairly large books, I highly recommend it. Although I haven't actually read the latest one yet.
While I'm recommending books, I think a good series to mention would be The Icemark Chronicles by Stuart Hill. A couple people I convinced to try them out got bored, but I was obsessed with it a few years ago. Ishaera would not be what it is without them (although this re-write is considerably less thinly-veiled plagarism than my 6th grade version was). Yes, I read them when I was 11, but I still love them now, so don't let that stop you.
I freaking love getting good books for a low price. It's just that I don't have the shelf space to keep all of them. And I just got new shelves.