Altair's part in the series

Assassin's Creed: Altaïr's Chronicles (PSP)
Assassin's Creed
Assassin's Creed: The Secret Crusade (book)
Assassin's Creed: Bloodlines (PSP)
Assassin's Creed II (10 minutes)
Assassin's Creed: Revelations

==========

Ezio's part in the series

Lineage (30 minute short film)
Assassin's Creed II
Renaissance (inaccurate novelization of 2)
II: Discovery (iOS game)
Brotherhood
Brotherhood novel
Project Legacy (Facebook game)
Ascendance (short film during Brotherhood)
The Secret Crusade (novel)
Revelations
Revelations (novel)
Embers (30 minute short film)
The Fall (mention as "one of the best Mentors" [AFTER Brotherhood came out])

==========

Desmond's part in the series (actual protagonist)

Assassin's Creed
Assassin's Creed II
Brotherhood
Revelations