There seems to be a fair amount of stuff available for using sketchup on the net which is free so my first question Joe would be what would your guide offer that's not already available? Not saying it's a bad idea at all, there are lots of tutorials for photoshop available, some are very good some are rubbish so a lot is down to the author.
Other thing is are there any copyright restrictions that would stop you doing it? Don't want to spend a lot of time & effort producing an ebook only to find you have to pay the software producers lots of money.