Neverwhere, Neil Gaiman.
This sees Gaimain in a more impressive mold. Strong characterization, great atmosphere, and a main plot that works to his strengths in exploring the invented world. For the most part the story is an excuse for wandering around and showing off the creativity of the environment, but the actual narrative makes a lot of sense and plays well in retrospect. But above all the book's success rests on the fact that the strange community living sideways to conventional reality is made so abosrbing, and that the mythological connections of this are so rich.
There's a great insight at the core here--basing notes of mythology off main monotheist strains, devils are often disturbing, but angels are simply terrifying. Anchoring the angel Islington to the account makes for a great contrasting presence--at first Islington is a cold ally, then shown as the larger villain, then becomes a very terrifying villain once the majestic self-control starts to slip and we see the ecstatic hatred boiling just below the surface.
This all feels better than most of Gaiman's YA output because it sustains its characters better, there's a sense of real danger for them accompanied by more significant growth. The range of the novel is simply wider than much of Gaiman's stuff, and it suits the construction of this work enormously. It also inhabits the conventional world more fulling, immersing itself in a number of scenes in aboveground London before the characters slip off the conventional straits. The effective hybridization of both make this urban fantasy at its best.
Similar to and better than: Gaiman's Coraline
Similar to and worse than: Mieville's Rat King