David Crystal describes the his preface the the what this book is. It is a collection of short essays, very clearly written, on 73 topic mostly headed "How....' They are carefully arranged but not designed for continuous reading. The essays are not "encyclopaedia entries", more overviews of the topic in hand... for example 'How Language Changes'. Students (and teachers) will find it a good starting point for a topic of interest. But there lies its weakness... It's bibliography is brief and it's index is conventional.
I tackled several of the essays which interested me. Some I followed up on Google. Others, which I casually dipped into, added to my stock of 'could come in useful' things to know. The book is worth a place on my shelves.