I too have enjoyed this companion book as it really delves into the back story of how these shows came to be. It will also inspire you to get the DVDs out and look for the continuity errors that you'd never notice without the insider information provided by this guide.
The episode guide is very informative and the A-Z of the supporting cast was also really interesting to read and provided a nice trip down memory lane.
I agree with the previous post that it would have been a nice addition to have more on Robins Nest and to read the cast memories from this show too.
All in all, this is a great read and a must buy for any fan.