Many of the varieties not currently available commercially can be seen in botanic gardens throughout California. As far as I know, this book is still unavailable in the US, so you will have to pick it up over there. It is available as hard or soft cover. Peter Olde is the leader of the SGAP's Grevillea Study Group, Neil Marriott is a Victorian nurseryman and former owner of the reknowned White Gums Nursery. This is a definitive work on Grevilleas, with three volumes.
Volume one discusses history, evolution, biology, and general cultivation.
The second and third volumes are an alphabetical treatment of all the species, from description to distribution and cultivation. Words cannot express how grateful I am to find they had the sense to include this most important feature usually found missing in books otherwise excellent. The only deficiency of this book is that it doesn't consider hybrids, although I have heard rumors that such a work is in progress. Encyclopaedia of Australian Plants suitable for cultivation, by Rodger Elliott and David Jones. Easily the most exhaustive and definitive source of information on Australian plants in cultivation, soon to be in cultivation, or with even a prayer of a chance of being introduced to cultivation.