For discussions of the notion of ‘correctness’ and the status of usage, see Quirk (1968), especially chapters 5, 7 and 8 and Appendix II which is by J. Warburg.
The inevitability of linguistic change, and the social and psychological pressures which bring it about, are the subject of Aitchison (1981).
Most introductions to linguistics contain a discussion and a rejection of ‘prescriptivism’ in an early chapter. Lyons (1981) is a good example. A book which attempts to bridge the gap between popular and academic conceptions of language study is Bolinger (1980).
Introductions to linguistics usually also contain discussions of dialect and register; for instance, Halliday, McIntosh and Strevens (1964), chapter 4. A small book devoted to the concept of register is Gregory and Carroll (1978). On dialects, see Hughes and Trudgill (1979).
For further reading on the analysis of English grammar, the most useful compendious treatment is Quirk et al. (1972), and its abridgement, Quirk and Greenbaum (1973). These contain a large number of references to works on particular grammatical topics. My own book, Young (1980), has a fairly detailed study of a relatively limited area. The amount of overlap with the coverage of the present work has been kept to a minimum.
Crystal (1980) is a useful source of explanations for linguistic terminology and is obviously much more comprehensive than the glossary of this book.