Betsy Clebsch knows loads about these wonderful plants, and she loves them too. You can sense her excitement when she says she's been able to include 50 or so new species and cultivars in this new edition (first published 2003, now out in a new paperback).
Most of the book is a typically great Timber Press production, following the standard format of going through the plants as individual species and cultivars, discussing wild origins, merits, tips for cultivation. Almost every plant is illustrated with a colour photo. Detailed hints on cultivars are particularly useful - S.leucantha "Santa Barbara" is more compact than "Midnight", for example. Under S. guaranitica, Clebsch gives Penelope Hobhouse's cultural advice for Britain as well as her own for North America. She's also good at recommending plant combinations, though I have to admit many aren't quite to my taste...
Reading this book I came to think of Betsy Clebsch as this really nice friendly woman who is sharing all her hard-earnt knowledge. Her little details about famous Salvia breeders and collectors give the text a lively quality that's surprising in such an encyclopedic book. If you like Salvias at all, and you like to know a lot about the plants you grow, you will really enjoy this book (and probably will quickly find yourself googling unusual ones to see where you can buy them... So potentially, an expensive purchase....)