It's easy to find material on certain historical persons--Arthur, William of Normandy, Alfred the Great, Edward I, Henry VIII, William Shakespeare, for example--and not so easy to find it on others. Gruffudd ap Llewelyn falls into the latter category. While I'm ill-equipped to say whether this book contains everything you'll want to know about Gruffudd, it probably comes close.
There's an awful lot of material in here, especially for its length. It's presented in a clear, organized way. It's also not terribly dry, which is something that seems to plague history books in general, and biographies in particular. Davies' bibliography is extensive, leaving the reader with a plethora of sources for further reading. Which, depending on one's interest, might wind up being somewhat redundant, as Davies does an excellent job of bringing it all together and sorting out the relevant material from his sources.
I was left with a much better understanding of Gruffudd's role in Welsh history, and particularly how Wales came to be subjugated by England. There's still an awful lot that I think I'll have to re-read later, because some of the presented material is kind of dense. I'd consider this a must-read for anyone interested in Welsh history.
The one thing I really wish had been included would be more maps! I suppose this can be forgiven, given an apparently British target audience. Which would be like putting a map of the eastern U.S. in a book about the Civil War, on the premise that most readers will know where, say, West Virginia and Kentucky are. But for the rest of us, maybe we don't know where Herefordshire and Mercia are, let alone how the geopolitical borders have changed over the centuries.