These legs are quite easy to make and very stable if done correctly - but they possess a bit of wobble as well.
That wobble came in handy when a car crashed through the art section of the bookstore I work at, however. The shelves fell when the car came through and the table merely leaned out of the way as the shelves hit it.
Invincible table for the win! (UPDATE: Crash pictures on the last step!)
I also apologize for the pictures in the instructable being hand-drawn. I did not document it as I went along and this is my way to make up for it!
Step 1: Gather your books!
Your best bet is Reader's Digest Condensed. They print these books in such mass quantities and no one wants them. I'm know every thrift store I've ever been to has tons of them. You'll need to remove their dust jackets if they come in them.
Otherwise, look for sets of books at thrift stores.
If the thrift stores fail you, check at yardsales. If you buy regular run of the mill fiction and nonfiction, keep in mind that the hardbacks come in two standard sizes - book club and publisher. Book clubs are normally a smaller size and shoddily made. Publishers will be the larger size and they will also have well reinforced spines (you'll see a little bit of cloth peeking out, in most cases.)
I can't really tell you how many books you'll need, though.
To determine this, decide how tall you wish your table legs to be. Keep in mind that you should keep them 30 inches and shorter in most cases - otherwise, you'll get too much sway!
Once you know how tall you want them to be, try to get an average thickness on the books you're considering and buy just a few more books than you'll need. Once they're compressed you might find you need an extra book to help them line up!