iLounge (neé iPodLounge... see a pattern here?) has a nice free pdf book on everything iPod. You can download it from their site or get it by clicking on the badge here.
I stopped surfing iLounge, because their site got too complicated and advert-filled. But they are still a good resource for information. Their speaker reviews are priceless or at least were when I was in the market a year or two back.
The iPod Book seems to suffer from a bit of the same problem as the iLounge site - the first 14 pages are mostly promotional material. In their defense, they had to pay for this somehow, I suppose. However, ads seem to be interspersed throughout much of the rest of the book as well. Ratings lists are tucked in the middle (p46-48), with the text discussing a few prominent products only. (Let's hope these are the top-rated, not the top paid advertisers.) The second half of the book seems to be devoted to tips about using iPods. Some of these are very useful - nice to see in one place.
What I like best is also my second biggest gripe about the book - the thoroughness. iLounge covered lots of ground with this - which is very helpful. However, I feel they often went too far, devoting space to too many truly extraneous things like Mac cases, style, and buying an iPod on eBay. C'mon!
Let's give credit where credit is due, however. It's a great resource, that is both free and comprehensive.