Most that needs to be said about Tokyo Babylon has been said by other reviews, it's a fantastic story with fantastic characters and a great emotional ending. What I'd like to mention however is in its relation to other series be Clamp, namely X1999 and xxxHolic.

This book serves as a prequel or prologue for X1999, providing backstory for (primarily) two of its side characters, while at the same time having nothing to do with X1999 beyond a few hints. It effectively works as its own standalone tittle they bear so little in common.

As for xxxHolic, while not directly connected they share the same plot structure and themes, that of an episodic story of helping people overcome their (usually) self-induced spiritual problems. It can and has been said that xxxHolic serves as a spiritual successor to Tokyo Babylon for these reasons. And in that respect they also quite often share the same tone aswell.