This is an awesome post. Some thoughts:

• You can get basically any even slightly popular book for free on the internet. You can use this to compare books before you decide on one to buy. You can get books either through torrents or (more easily) through file-hosting services. For the latter, you can usually find what you're looking for easily by googling the author's name, book title and 'rapidshare'. This is probably illegal.

• Interest can be a useful heuristic for deciding on books, since it filters out things too far above or below your level.

• Another heuristic: whenever you read anything, keep in mind that, a year from now, you will almost certainly have forgotten almost all of it except what you put into spaced repetition. I find it ridiculous that so many people, a few years after getting out of school, have barely anything to show for it.

• Yet another heuristic. Pick a field you like and use, say math. Realize that you will never know all the math there is to know. So whenever you read a book, ask yourself whether you would really prefer to know this over more math.

• Amazon reviews of books are usually brilliant and show at a glance how well-written and controversial a book is.

• Perhaps reconsider whether you need to read fiction at all. If everybody else thought immersing yourself in imaginary worlds was some strange perversion, would you still do it?