A superb reference book covering all aspects of the development of the English language, its vocabulary, usage, orthography, spelling, variants, punctuation etc.
David Crystal wears his expertise lightly, which makes the book a delight to use and very accessible, with lots of illustrations, diagrams, photos and examples. There are also appendices and lists for further reading.
In general, Crystal is a pragmatist who loves language; he is certainly not an authoritarian pedant. He accepts and explains language change, explaining controversies such as whether or not it's acceptable to end a sentence with a preposition (it is) so that readers can make up their own minds, aware of the implications of whichever choice they make.