I've been thinking about this since Jeff told me you posted. My books are all outdated from almost thirty years ago. Look for the most up-to-date edition of Book of The Dead. I'd also start with resources from the Egyptian museum and Archeology Today.
I've found Wikipedia to be good on Norse myth, as the articles often discuss how we know, and variations in the sources.
It wasn't until hieroglyphics were deciphered in the 19th century that it became possible to read Ancient Egyptian literature. Plutarch's On Isis and Osiris is a rare account by a Classical author of Egyptian myth. Wikipedia's article Osiris myth compares it to the Egyptian sources.
Art can be an important source of information about myth. It sometimes confirms that such-and-such a myth was current at such-and-such a date. There's a famous book by Othmar Keel called The Symbolism of the Biblical World which relates the iconography of Ancient Near Eastern art to the imagery of the Book of Psalms. Back in the day my local library had a copy I meant to read and never did.
A point to remember is Egyptian civilisation lasted an enormous amount of time. There must have been significant religious changes. The pharaoh Akhenaten attempted a religious revolution.
Egyptology is an area where knowledge advances. I believe DNA studies have cleared up the long-debated identities of some royal mummies.