If you want to get a book about English hallmarking, Bradbury's Book of Hallmarks published by the Sheffield Assay Office is a long established reference.Make sure that you get a 2014 or later edition, because only those contain correct information about British import hallmarks applied to watch cases.
I don't make any attempt here to cover manufacturer's trade marks, of which there are thousands.
If you want to identify a trademark, an invaluable resource is Mikrolisk.
In 1478 the first permanent assayer was appointed and items had to be taken to Goldsmiths' Hall to be assayed and marked, the origin of the term "hallmarking".
A system of variable letters, changed each year when new wardens were elected, was introduced to identify when, and therefore by whom, an item had been assayed.
Hallmarking is carried out by an organisation independent of the manufacturer of the item.
In addition to the fineness, hallmarks can show where and when an item was hallmarked, and who submitted it.
This page isn't an end in itself, it is intended to help make a start on identifying the hallmarks in your watch case and then lead you to another page with more detail.
On this page there is a brief description of a number of different types of hallmarks that you are likely to find in a watch case, and then for the British and Swiss marks there are links to take you to the full page of information for that type of mark.
Swiss watch cases were required to be assayed and hallmarked from 1880 but the identities of Swiss watch case makers are poorly documented before 1925, after that date all Swiss gold watch cases had to be stamped with a mark that identifies the maker.