The best way to learn photoshop is to mess with it a lot. Once you learn a couple tricks with it try comparing notes with someone else that knows it and learn some new ones. There are so many features in that thing that you will probably never learn them all but you only need a pocketful of them to do some fun stuff. My suggestions for things to experiment with are:
-just spend a little time tinkering with each of the tools in the pallet and seeing what they do.
-try different stuff with the 'layers'. A lot can be done with them so poke around with them and see how it looks
-the color picker tool is your friend. try and get good with that. Shift-click can be used with that to select lots of stuff you want to get rid of (or copy out to some place else). I have seen some online guides but mainly they were specific to doing a certain operation or task. Like here
http://machall.com/indexcontent.php?doshow=info_art where a webcomic artist tells how he builds up the comic in photoshop from pen&ink drawings. Getting a book on it does not hurt but most of the learnin is by messing with it. Once you feel like you have some stuff figured out then a book or guide is a nice way to learn even more tricks (since now you will know what they heck they are talking about).
Layers is where I have trouble. I can do things with fixing exposure on all or part of it, I love the clone stamp tool, but I do it all on the same layer because I find the layers too complicated... ecurbh ~tries~ to help me.