Just about a year ago, I found this great download from the website of Mark Johnson. Specifically covering Photoshop CS3 - I bought this and have made great use of it.
Well I see he has now added the companion for CS4 which can be found here. I have posted the link for the download version, but there is also a CD version available if you prefer, although it will cost you a little more.
They are big files, so be warned.
If you are users of either version, I would really recommend purchase and download - it really is excellent, and will give you all you need to know in one easy volume.
It is great to read off the Netbook while carrying out the work on the main desktop.