the basic idea is that there are a set of immortals who deal with the guidance and collection of mortal souls and each book focuses on a particular "incarnation". the first book is death, second is time, etc. the overall story line is that satan and god are battling for the collection of souls and each book shows how the most recent incarnation has to deal with satan's manipulations of their office. the first book, "on a pale horse", is a great read. the rest all tie in to the concepts presented in that book in some fashion as well.