I've been very happy with Secret College. It might not be what you expect if you're looking for the traditional treatment (PrC's- in fact, there are no PrC's in the book). It offers two new core classes which fit the theme quite well. It's loaded with new spells, has some great creatures (though not enough, IMHO), lots of interesting necromancy rules/exposition (how to collect bodies, etc.), and a sample secret organization of necros to plug into a campaign. The book is very well done, and makes necromancers a class to be feared as villains.
I also like Gar'Udok's, though not as much, but the price is right in PDF format. I don't have the Mongoose book, so I know little about it.