If you're going to get into the extra-biblical books, 1 Enoch is the place to start. This book was held to be scripture by lots of ancient Christians -- including Jude, who quotes from it -- and by Ethiopian Christians to this day.
If you don't want to get 1 Enoch (it's called 1 Enoch because there are a 2 Enoch and a 3 Enoch; 1 Enoch is also the Ethiopic Apocalypse of Enoch) in a big anthology, there's a recent standalone translation by George Nickelsburg and JamesVanderKam. The same guys also have a two volume (one and two) commentary on 1 Enoch, well worth the read.
Margaret Barker's first two books, The Older Testament and The Lost Prophet, are about how the earliest Christians new a different Old Testament than we have today... an Old Testament that included, among other things, 1 Enoch. More about Margaret later.