The Magic Lantern series is an old faithful - thorough and understandable. This guide is likely best suited for those new to Nikon DSLR's. It would also be useful to intermediate users. The awesome D90 performs so well out of the box that a guide like this is not necessary, but it is well worth the money.
Strong points include explanations of: focus, metering, flash, custom settings.
If you are relatively new to digital photography, a competing guide - Nikon D90 Digital Field Guide - contains much of the depth in this Magic Lantern Guide plus a good deal of solid advice on general digital photography technique. For those in this situation, if you were to buy one D90 book only, I'd go with the Field Guide.