[I received a copy of this book through NetGalley.]
Technically speaking, I could only review a PDF copy, not a physical one, so I can’t comment on the quality of the paper and of the printed colours; but the photos in the digital copy were, at any rate, vibrant and gorgeous.
This book presents quite a few parks, gardens, walks and cycling paths throughout Greater London, ranging from cozy gardens to cemeteries and preserved areas—we all have that idea of "the big city" as made of concrete only, but London is actually a pretty green city, or at least one with many more green places that one would suspect. Along short texts and beautiful pictures, the author also takes care of giving addresses, opening days/hours, and the closest Tube lines, to make it easier to find these locations and book entrance when this is necessary.
While visiting more than a couple will be difficult for someone who's in the capital for only a few days, if you're a local, or go to London more regularly than just every few years or so, "Outdoor London" will definitely give you plenty of ideas for both summer and out of season outings. (I was particularly happy to find about several smaller gardens in the City, since this is a very accessible area for me, and now I have no excuses whatsoever not to go visit these places more often.)
If anything, maybe I would've liked to see more details about each park or trail—a few more anecdotes, perhaps? But that doesn't detract from the book as a whole.