You folks don't know lucky you are. Once upon a time there was just W C Haycraft and Pitman's Motorists' Library.
Haycraft was good for the basics, but shied away from anything more complicated than a Top End Overhaul.
Haynes is pretty good, but there are mistakes here and there, so it ain't always the true gospel.
Roy Bacon's Twins and Triples is frustrating as he will give note of the variation in parts, but not illustrate them specifically. The gearbox is typical, he will say a gear or shaft changed, to improve selection, but not describe or illustrate the change, so you can't tell if you have the correct or spurious part. The oil pump changes are similar examples.
I have some original parts books, but have invested in modern reproductions for regular use, rather than subject the original greasy ones to more use. Seeing how things go together makes life a whole lot easier.
Draganfly, although coming in for a bit of stick as regards their spares business, have a great online parts catalogue, often less effort that finding and opening the book.
Brightspark and Priory Magnetos are my choice for electrics. Literature sections here are very useful for identifying what you have.
Don't forget the Literature Section of this very Forum, and for a view of how the other half lives, try "Old Thumpers", a US based cornucopia. BSA Service sheets are detailed and there is an illustrated catalogue of BSA Special Tools. Not that you can buy them, more to use as a basis for fabricating your own to overcome some conundrum. Site is easy to navigate, and well worth a look.
Swarfy.