Here you'll find anthologies which feature numerous authors.
For contiguous stories with multiple collaborating authors, please look under Collaborations.
Books are listed ALPHABETICALLY BY TITLE and I've included some pointers for series and franchises.
B (DC Comics)
C (Marvel Comics)
D (Doctor Who, Dragonlance)
H (Hellboy, Marvel Comics)
I (Indiana Jones)
L (Dark Tower, Discworld, Sword of Truth, Alvin Maker, Majipoor, Earthsea, Memory Sorrow and Thorn, Song of Ice and Fire, Pern, Riftwar, Wheel of Time, Realm of the Elderlings, Outlander, Otherland, Symphony of Ages, American Gods, Shannara)
M (Mars Attacks, Marvel Comics)
O (Marvel Comics)
P (DC Comics, Predator, 2000AD)
R (DC Comics, Marvel Comics, Warhammer)
S (Star Trek, Star Wars)
T (Doctor Who, Discworld, Earthsea, Lankhmar, Hitch-Hikers Guide to the Galaxy)