Endgame was part of the "Earth Arc" -- a series of six books where the eighth Doctor, after destroying Gallifrey, is stranded on Earth for about 130 years without his TARDIS and doesn't remember anything about himself or his past/origins except that he's known as the Doctor. The six books were all standalones, taking place a decade or two after the previous one; there's no continuity between them (to their detriment, imho). You can safely read Endgame by itself.
