(as opposed to the hardback, which is really a conversion of return to)
KG, do you have access to the original ToH, you can see the tiny bit that is there. (also nice for the illustration book).
Return to by Bruce Cordell is certainly an expansion and adds quite of background (and skull city, wich you know from the new 4E hardback)

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.