I'm an avid reader. I had to resort to keeping a list of what books I've read, and those I want to read. I previously have noticed myself re-reading a book. I'd often get 1/3 or more through before realizing that I've already read the book.
I started keeping a list of books that I've read a long time ago. Originally the list was a text file. Then, I upgraded to a Microsoft Word file. The next upgrade was to XML. I now maintain the list in XML and use XSLT (and sometimes XQuery) to convert it to other formats: mostly HTML or CSV.
I've also been experimentally converting it to an SQLite database with the goal of being able to read it on my PDA (HP iPAQ 2215).
Here are some links to my book list (in two different forms):