One of the better anthologies on revolutions is the third edition of Revolutions: Theoretical, Comparative, and Historical Studies edited by Jack Goldstone. The anthology contains excepts from various theorists trying to answer just those questions and is an introduction to the various explanations and models. Goldstone's Revolutions: A Very Short Introduction is a bit cheaper than the anthology, and shorter, but is another good introductory text. Theda Skocpol's States and Social Revolutions: A Comparative Analysis of France, Russia, and China is getting quite long in the tooth, and her assertions on the Chinese Revolution have been superseded by later scholarship, but is one of the landmark texts for a structural approach to understanding revolutions.