Just barely three stars.
A Compilation Book drawn from his first three books in the "Source Book" series (The Druid Source Book, The Celtic Seers' Source Book, and the Bardic Source Book), Secrets of the Druids is a scaled back (abridged) tour through the trilogy.
This text covers writings from the Classical commentators, Celtic Folklore, various stories and interpretations regarding the Druids, Seers and Bards of Celtic & Gaelic tradition, in that order.
Since this book is a compilation geared toward a general audience, it lacks many of the footnotes and details in the original three books, so you have to refer to the original texts to find many of the references or sources for the material. There is also no index, which is rather annoying, as it makes it very difficult to find material in the book when you're searching for it.
John Matthew's translations are mostly reasonable, but some of his interpretations of these tales are a little far reaching. If you can put aside some of the commentary, and don't mind the other issues already mentioned, there are some interesting stories presented, but I've seen much better books on the subject.
"Secrets of the Druids" is geared for a general audience, rather than for scholarly study/reference, and it highlights what the author considers the best of his works in one concise, easier to read source book. As it stands, there is a lot of interesting information in this book, but you have to search elsewhere to find out where much of it comes from and you can't rely on Matthew's interpretations.
I definitely prefer the individual three source books over this compilation for the footnotes and index this text lacks (with their own inherent limitations considered), but, if you don't mind doing further research, and can keep track of where everything is, it's decent (just).
Unfortunately, if you've already read the other volumes, you will also find little new material is covered in this compilation. It's a compilation, nothing more.
All and all, this text may be enteraining for someone new to Druidry and Celtic Folklore, but for those who've studied a bit more, it's really lacking.
As a fairly serious student of John and Caitlin Matthews many works, I must say I was rather disappointed by this latest one. As has been previously stated, the material has already been covered in his three volume series of sourcebooks. Besides that, the material is, in my opinion, poorly arranged and incredibly dull. Add to this the fact that many of the sources are questionable in both authenticity and scholarly value, and you have a poor book from an otherwise outstanding author. If you have need of an impressive looking reference book, then by all means pick this book up. If you're looking for an exciting read on the Druids, this is not it.