In more depth:
This purports to be the story of the interactions between a scholarly convict at an asylum for the criminally insane and the long time editor of the first edition of the Oxford English Dictionary.
The problem is that the book is not, and indeed really can not, be about this. The two men did have a long correspondence, and did meet from time to time for over twenty years, but the character and content of these meetings is not anything particularly spectacular. Mostly, they were collaborators on the great project being directed by the professor, which the madman was a (copious) contributor to.
The madman made his mark on the dictionary by specializing in finding good quotations for difficult words. He did this by using his twin resources of an exceptional library (afforded by high station, a pension, and privileges), and the copious amounts of time afforded him by being locked in a lunatic asylum for the criminally insane.
But, really, the interactions that produced these contributions are not particularly of interest. The professor queries the madman about a particular problem, and the madman frequently delivers a good solution. Not much more to it than that, except maybe for the hardcore lexicographer.
Instead, the early part of the book especially is full of wonderful descriptions of Victorian England, from Oxford to the rough streets of Lambeth. That part is a wonderfully quick read that does a good job of establishing a sense of place and time.
But after that it bogs down, because it becomes a biography of the madman, especially the period during and after he begins his contributions to the dictionary. There are some interesting parts to this, especially at the beginning when his circumstances at the asylum (where he lives most of his life) are described. At the same time, he is making his greatest contributions to a dictionary project that looks impossible to complete, and is not receiving widespread accolades or public support.
However, the last 50 pages or so, covering the period of time after the madman has become incapable of contributing to the dictionary due to his declining physical health, are mostly just sad. There really isn't much to tell, and the one bit that is supposed to be suspenseful, while somewhat surprising, is not delivered well and didn't hold my attention.
Another substantial problem with the story is that measuring the madman's influence on the dictionary project is difficult. Certainly, for the attention given, the madman would be expected to be a major contributor, and the author includes some notes from the professor/editor to other colleagues that indicate this is the case. Indeed, one of the early volumes was even dedicated to him. However, the author also points out that there were other contributors, as well, that matched the madman at least in volume. And the size of the project is such that even the greatest individual volunteers accounted for only a few percent each. So, then, does this man really deserve the credit he's given? Does he deserve the book's implication that he was enormously important in ensuring the publication of the work? I think it's hard to say yes. Even towards the end of his own life, the madman's story and contributions were already being romanticized, and while the author is somewhat wary of that, limiting the focus of the book as he does makes it difficult to dispel more than the factual basis for the myth.
All in all, a good start, but runs out of steam down the stretch. A quick read, too. Would have been better as a shorter work, and more evenhanded. I can't recommend it, but I wouldn't say it was that bad, either.