I think this book is absolutely fantastic! It makes you think about things that are obvious to the extent you ignore them, normally. It has charts on all the instruments, and on the charts he further develops by saying which part of the instrument is best for what type of tone. For example, with an Oboe, the lower part has a much more dark, rich and nasal sound, but the higher up you go it becomes more piercing. Although these seem like simple concepts, when put in context Rimsky Korsakov's book is incredibly well written. Do bare in mind not all of it is his own work, it is pieces of his own work, completed by a friend of his.
Nonetheless this is a brilliant book, a must have for anyone wanting to study a bit of orchestration!
PS. It's such an easy read, unlike some books that are just complexed English with one actual meaning! >.<