Frogs, flies, and dandelions
The book starts by looking at the question of how a species is defined - sometimes it isn't very clear cut. Schilthuizen goes on to look at the various theories of how a new species might arise from an existing one. Isolation of a population is one possibility, and if it is a small population then there is a greater likelihood of significant genetic variation. But since the ability to mate is central to the definition of a species, sexual selection can also play a part. Sometimes new species can appear fairly suddenly, and it can be hard to deduce what caused them to split off from the original species. The later part of the book looks at examples of this in more detail.
The book can be read by the non-specialist, although I didn't feel that it was quite as lively as the blurb suggested. It's certainly worth reading if you want a better understanding of some of the puzzling features of the occurence of new species.