After London is made up of two distinct parts. First is "The Relapse into Barbarism", which describes the decline of civilization, but more importantly the recovery of nature, after an unspecified disaster. This section draws heavily on Jefferies background as a nature writer, and is essentially a detailed thought experiment on what would happen to the English countryside without many men around. For a potentially dry topic, it is surprisingly readable - largely because Jefferies describes the reaction of each aspect or species plausibly, then moves on without bogging down in details. Normally, this kind of material would exist mainly as backdrop for the characters. Here, one gets the feeling that the second half of the book was written mainly as an excuse for this imagination of the environment.
The second part of the book "Wild England", is a more standard adventure story about a sullen and disaffected young noble and his search for a place in the world (one that will impress his beloved). The story is simple, and still relies heavily on descriptions of the environment as the hero travels around. But it is again well thought through, and the hero's emotions are as plausible and realistic as the scenery around him.
The book is a pleasant read, if not exciting. I would have considered giving it a higher mark, but for the fact that the story effectively stops mid-stream. We can imagine what happens next; it's not essential that we're told. But because there's no gradual letdown, it feels very abrupt - enough so that on reading an e-copy a couple of decades after the print version, I went looking on the internet to see whether I had somehow been shortchanged.
So, a fun, interesting read, but a little disappointing at the end.