If you teach English Language A Level, I’d recommend the following online resources:
The relatively new site The Grammar Teacher, maintained by Dick Hudson and Geoff Dean has lots of helpful (and recent and positive) material about grammar.
As for books:
David Crystal’s “Cambridge Encyclopaedia of the English Language” is great to have in the department.
Deborah Cameron’s “Teachers’ Guide to Teaching Grammar” is excellent if you’re less confident on word classes and syntax.
The “Language of ...”, “Living Language” and “Intertext” series are good for student reference, especially for Language Investigations.
For the AQA spec, the OUP textbook (by authors of the old Nelson Thornes ones) is also particularly good :)