don't combine whole series and pass it off as one book.
lotr is 3 books, 6 parts.
harry potter #5 isn't all that long. Just looks big cause of weight, page thickness, margins, font.
PLUS it isn't all that good.
if you wanna combine series, try
Jordan's WoT
Wurt's wars of light and shadow
Goodkind's series beggining with Wizard's first rule
hehe
apart from that, long books (single books) = Melanie Rawn. huge books (1300pps'ish), lol, tiny writing, thin pages. :-\
few others, but don't exactly remember names.