I've recently acquired the monumental Flags Through the Ages and Across the World (1975), by Whitney Smith, Ph.D. considered the founder of the "vexillology" (by the way, this word was created by him), the study of flags. It has more than 350 pages, 2257 artwork pieces made exclusively to the book, , featuring the masterpieces by Franz Coray, and many selected deluxe illustrations and photographs.
It's certainly among the best vexillology books ever written, and, although out-of-date and with minor errors, it's worth any money you can invest on it. The books excels both as a guide to vexillology and as a reference book.
This book is divided in some sections, and I'll describe them one by one.
Introduction (pages 7-31). Contains a brief introduction to the vexillology, and a very complete glossary of terms related to flags and heraldry.
Flags Through the Ages (pages 32-203). For its historiographic valor, the highlight of the book. This section tells the history of the flags since its ancestors ("vexilloids") from five millenia ago until the current usage, including sub-sections related to some historically relevant flags (like the standards of Charlemagne and Joan of Arc), the evolution of flag etiquette, the usage in many contexts (religious, military, naval, etc.) and timelines to some of most famous national flags.
Flags Accross the World (pages 204-303). Describes and explains the flags and coats of arms of all independent nations (and some subnational entities, when relevant) at the time of book conclusion. In the course of time, many of them became obsolete, but this section is yet a relevant text for reference. The section contains appendixes about international flags (like the United Nations and Red Cross) and flags of some ethnic and cultural minorities.
Symbols (pages 304-348). In a very original and intuitive layout, the section cites many flags with common symbols (geometric shapes, animals, plants, etc.) and patterns (the crescent and star, for example).