The Book of the Dead was called the Book of Going Forth by Day in Egyptian, and is often found buried in tombs from the New Kingdom period of Egyptian history onwards. It is a collection of "spells", some of which are derived from older texts found in Middle Kingdom & earlier tombs (the Coffin Texts and the Pyramid Texts, respectively). Taken together these spells would give the deceased the best chance of navigating the afterlife. Each version of the book seems to have been unique, with a different selection of spells, although over time this became more & more standardised. There is some split in opinion amongst Egyptologists about whether the Book of the Dead was read by the living, or just buried with the dead (and thus "read" by them as they went through the afterlife where it would be useful) - Kemp comes down on the side of it being read by the living, pointing out that there are spells that say things like "And for him that knows it on earth [...] and he will proceed to a very happy old age".
Kemp quotes a handful of these spells and uses them to provide an insight into how the ancient Egyptians thought about the world around them, about the afterlife & the gods and their place in the universe. The mindset of the Egyptians seems pretty alien to someone brought up in a Christian religious & cultural environment - there seems to be no necessity for consistency, for instance. The myths don't necessarily form a cohesive story when taken all together, they are fragments which illuminate some aspect of the truth behind the universe. There seems to have been no dogma & insistence on everyone believing the same thing as each other at all times - in fact, Kemp brings up the fact that belief is never mentioned, you do these things, you say these things and it will work out in your favour. Belief is almost a given, and certainly doesn't seem to have been focused on. There are also multiple things that happen in the Egyptian afterlife that are mutually incompatible - for instance there is a (well known) spell which has a list of 42 denials all of the type "I have not done bad thing". And there are other spells which are to ensure that one's heart does not bear witness against one and admit the bad things one has done. And it doesn't appear that you were encouraged to lie, instead in Kemp's words "The Egyptian view allowed for two contrary streams of thought, one which denied that any wrongdoing had been committed and one which sought ways of avoiding punishment for the fact that it had.". Some of the afterlife is an endless journey accompanying the sun as it travels beneath the land between sunset and sunrise, some of the afterlife involves being able to go forth into the land of the living in various forms, some of the afterlife is your trial by the gods, some was in the Field of Rushes doing the work that must be done to provide food. And it was all simultaneously true (as I understand it).
I found this an interesting and very readable book - as I write this little review I keep wanting to say "oh and another thing it told me that's neat", it's just full of little tidbits of interesting stuff. I'd definitely recommend reading it if you're interested in Ancient Egypt & the Egyptians :)