It is the best book on peonies that I could find.
I also have Peonies by McGough, which is good on pictures because her husband illustrated it . He is a professional photographer. One draw back is they do not actually grow peonies, but did do a good job of researching peonies. They are in New Zealand so the varieties are different from the ones in the Page book. Page is American.
I also have the Halda book on The Genus Paeonia. It is more on species peonies.