A friend of mine gave me this book for my b-day (merci Vincent!). It came to me highly recommended as he swears by the Magic Lantern series.
Unfortunately, this was the one I least appreciated of the four books I read about my D90 (yeah, I know, kind of obsessive, don't you think?). First half of the book is OK. Just OK. The second half of the book kills it: it's a repeat of the owner's manual.
I recommend David Busch's "Nikon D90 Guide to Digital SLR Photo" or even "D90 for Dummies" (for those who are really really "green").
Sorry Simon Stafford, your book didn't do it for me.