I read a bit of everything when I have the time (which is not much, I used to be an avid reader, not anymore :() so I can jump from some geek stuff (like "A brief History of Time by Stephen Hawking or Classical Mechanics: The theorical minimum by Susskind) to the stupidest/simplest you can think of.
As yourself though, I feel sometimes too tired to read serious books then I tend to read thrillers and mystery books, epic fanstasy. Those get my mind out of work and relax my brain. I love writters like Ken Follet (not in fav list but he's got some good books), Michael Crichton, Margaret Weis, Tracy Hickman, JJ Tokien (of course), Robin Cook... I could go on forever.
Although when I'm trully knackered and exhausted, I'd rather have a romantic novel. I can read them without concentrating at all (mysteries and thrillers may have you guessing and epic fanstasy... well, you really have to remember each character and keep their tracks throught the book!). You know how it starts, you know that there's a happy ending and really, most of the time you know what goes in the middle.
I have a little problem with reading and that's one of the reasons that I don't read more often... I need to read the whole book in one go. I can't read a few pages and keep the rest for tomorrow. I really need to read the whole thing.
And that's one of the reasons why I have a huge list of books that I wouldn't mind reading... and that I will probably never read. It is a shame and I did try to get out this bad habit so I could get more books out of my outstanding list but I simply can't. I do need to know what goes next. Sagas make me suffer so much if I don't have the complete series of books! :lol:
Edit: I do love sci-fi. And of course I introduced myself into the genre with the classics, starting with Isaac Asimov