Steve Lewis' book is an invaluable guide to climbing the mountain. I was teased as being a sucker for buying the book, and while it is true that all the information in the book can be gotten on the web, it is great to have it all handy in one place, especially as it lists the addresses and numbers for the ranger stations and the outer equipment stores.
The book is also a fun read and I think mentally helps you prepare for this difficult ascent. I highly recommend it if you're planning to climb Mount Shasta.