almost everything i know i discovered on my own, by just playing around with it
but it gets really tricky, like you can use series of tools and effects to get some kind of a result , for example someone showed me how to make a perfect lightning not using the brush tools - only applying different effects. and that's just something you can't really discover on you own so you do need the book .......
I learned by just messing around with all the tools, seeing what works best for what, and as Vicky said there's some effects that can be achieved simply by applying filters and the like. The most dangerous thing about Photoshop though, is deluding people. Apart from the misperception that having a higher Photoshop version makes you better at graphics (I'm most comfortable with version 4), a lot of people also fall into the trap of using cheesy filters like the lighting effects and lens flares :fero:
I think one of the most overlooked set of tools in photoshop are the image adjustments, if you learn to control those, it really helps no matter if you're editing photos or doing digital art