Inevitably with a publication of this kind, the selection of terms to be included will be the personal choice of the author. In this case, it is based both on my experience of teaching about language for over thirty-five years, and on my perusal of other works of a similar nature. The latter has been necessary especially where my expertise is limited, since no linguist has a detailed knowledge of all the areas of language study.
The terms have been selected with the beginning student of language or linguistics in mind, perhaps undertaking one of the GCE A-level courses in the English language, or in the first year or so of a degree course in linguistics, the English language, or a modern language.
Rather than being presented in a straight alphabetical list, the terms have been arranged under headings representing the traditional divisions of linguistics, and which may correspond to modules or courses that you are studying. The largest of these sections is for grammar (morphology and syntax), which reflects both the centrality of this part of linguistics and the fact that many of the other linguistic disciplines depend on and extensively use the terms of grammar in their own studies.
There is an alphabetical index at the end, in case you cannot readily find the term you are looking for; and there are extensive cross-references in the entries (in bold ), so that you can see how terms are related to each other.
A reference work of this kind cannot expect to be comprehensive. The hope is that you will not be disappointed by not finding the term you are looking for. If this is the case, however, please inform me, via the publishers, so that the term can be included in any subsequent edition.