I like reading the occasional historical fact (rather than historical fiction) "novelette," and The Professor and the Madman was definitely easy to get through. One can learn much from books like this, particularly the way normal people lived their day-to-day lives in a certain time and place.
A few things I liked about this book:
1. One will assuredly learn a thing or two about the English language, in reading it. You will learn some obsolete words, the origin of some words, and just get a refresher of other, more common words. Each chapter begins with a dictionary entry of a particular word, some very normal words, some more exotic words.
2. The parallel lives of the two main characters are interesting to follow. One feels real emotions for both. There are a few shocking moments in the book, which stand out quite a bit in front of the otherwise fairly tame narrative.
3. I grew up with the Oxford English Dictionary, and I always wondered how they compiled all the words. It was great learning about how they did that.
4. The book covers an array of themes and topics, and a fairly diverse geography. Mental illness, civil war, sexual propriety, crime and punishment, one can learn a little bit about a lot of issues in the reading of Simon Winchester's book.
I wouldn't recommend the book to just anyone, though. It can be kind of slow, and sometimes one simply grows tired of bouncing back and forth between the two main characters. It is also fairly short; one sort of wishes for more detail on certain events. In some places, the book reads like a crime/detective novel from the 19th century, in others it is more like a biography. It sort of skips around from one style to the next, almost as if different parts were written at very different times by an author in very different states of mind. Overall, though, this book is a nice, quick read, a good plot, and you will learn a thing or two from it.