To many people Neil Gaiman is best known for the Sandman series of comics. Some would say the comics are his best work, and others prefer his telescripts or his writing for younger readers, but if you are looking at his novels written for adults, the standout one is perhaps American Gods. Published in 2001, this has been a top seller ever since and has also been a hit with critics - which isn't always the case.
American Gods tells the story of Shadow and Wednesday, two criminals who travel together until Shadow finds out that Wednesday is actually an incarnation of the Norse god Odin (for whom Wednesday was originally named). The idea is that gods exist if people believe in them, so in America belief in the old gods and magic has declined as people have instead started to put their faith in money, technology etc.
Shadow and Wednesday attempt to bring back belief in the old gods.
You can find the book, as well as some reviews and comments, here.