Series Two is one of the Related Worlds. One world described in The Lives of Christopher Chant says that there is a world in Series Two that has a maze of bridges over rivers and streams. In those streams live giant eels. Later in the novel it turns out that eels flesh is a valuable magic item and is being regularly smuggled into Twelve-A.

The Related Worlds
Series One, Series Two, Series Three, Series Four, Series Five, Series Six, Series Seven, Series Eight, Series Nine, Series Ten, Series Eleven, Series Twelve