Step 1: Research and List
Try to find what genres of books interest you most and find titles that really embody the genre and write them down.
Listen to any and all recommendations from people whose opinions you trust or have reason to trust and write them down.
Do you really like movies? Why not check out the predecessors to your favorite films and.... write them down!
I suggest using a small piece of paper that can easily fit inside a wallet or purse or pocket so that wherever you are, you are prepared.
Step 2: Your Best Friend
Get used to this word because it is your new best friend! Immediately go to Google Maps and search for "used books" and your city. Go visit the locations, make friends with the owners, and browse their varied collections of books that should be much cheaper than the newer versions and not a whole lot different. If they are not very cheap, haggle. The book industry is not exactly booming and I'm sure that the store owners will appreciate any business they get.
Step 3: Your Next Best Friend
With more than 2,200 locations around the world, you shouldn't have too much difficulty finding one. If Goodwill is not accessible to you, there are countless other thrift store options to choose from. And not to the fact that a constantly changing inventory can mean a new load every week, and new chances.
Step 4: Another Good Friend
There is a whole section devoted to books as well as a section to post any books that you specifically want. Because Craigslist is community driven and run, you have no need to pay for shipping or any such thing, but be aware that it is always best to bring someone else with you when you meet the seller and try to meet in a public place, like a local Starbucks or Library :)
Step 5: Your Humbling Best Friend
There are some who view garage sales as "below" them, but if you are reading this Instructable, they aren't. If you want to be shrewd with your scarce funds, then utilize this very valuable source. "Garage sailing" is a long forgotten art to many and you can find much much more than just books. It's also a good excuse to hop on a bike and get some exercise while searching for new novels.
The main idea behind all of these steps is this:
One man's used junk is another man's treasure
Step 6: After Everything Else...
Warning: This step is not for those who are very picky about lending out their things. Enjoy :D