The Languages of China - S. Robert Ramsey This is a perfectly nice (if dated - 1987) book about exactly what the title suggests and it probably could only appeal to readers like myself who are fascinated by how people speak but aren't linguists or philologists.

If you are a linguist, I think the book is too general as it's more a survey of Chinese languages than an in-depth analysis of any one of them.

And because it was written pre-Tiananmen, pre-fall of the Berlin Wall, pre-Soviet collapse, I don't know how current Ramsey's discussion of the PRC's policies toward minorities is or the status of some of the smaller languages (some of which had only a few thousand speakers in 1987).

While it's not a spell-binding read that will keep you rapt for hours, I did find it fascinating and would recommend it to the interested.