Assassin's Creed Brotherhood on PSP Go? Not Possible

Assassin's Creed: Brotherhood was never released on PSP or PSP Go. It launched exclusively on PlayStation 3, Xbox 360, and PC (2010). The PSP hardware couldn't handle its engine (Anvil), open world, or multiplayer.

Assassin's Creed Games Actually on PSP

• Assassin's Creed: Bloodlines (2009): Direct sequel to AC1. Plays similarly but smaller scale, no multiplayer. Canon story.
• Assassin's Creed: The Secret Crusade (2011): This is a novel by Oliver Bowden, not a game. (Sometimes confused due to similar naming).

Note: Assassin's Creed: Revelations and III also skipped PSP. The Vita got Assassin's Creed III: Liberation (later ported to PS3/PC/HD).

For portable AC, Bloodlines on PSP or Liberation HD on Vita/PS3/PC are your only official options.