This encyclopaedia is simply brilliant for language lovers. It is clear and concise, brilliantly illustrated and laid out, and it makes the subject matter interesting. Once again David Crystal has managed to convey his love of language.
This encyclopaedia covers a wealth of information:
Part I. The History of English: The origins of English; Old English; Middle English; Early Modern English; Modern English; and World English.
Part II. English Vocabulary: The nature of the lexicon; The sources of the lexicon; Etymology; The structure of the lexicon; and Lexical dimensions.
Part III. English Grammar: Grammatical mythology; The structure of words; Word classes; and The structure of sentences.
Part IV. Spoken and Written English: The sound system; and The writing system.
Part V. Using English: Varieties of discourse; Regional variation; Social variation; Personal variation; and Electronic variation.
Part VI. Learning about English: Learning English as a mother tongue; and New ways of studying English.
As you can see, it's wide in scope so each section is concise and to the point, brilliant for referencing or just dipping in and out of for something interesting.
I would definitely recommend this to English language students and anyone with an interest in the subject.