I have the PDF that was expanded into this print product. I also have AEG's Undead, Mongoose's Necromancy: Beyond the Grave, and Green Ronin's Secret College of Necromancy.
I would describe is mostly like Green Ronin's work. A good amount of crunch, but with a lot of flavor text. If all you want it generic necromancy stuff, it is take it or leave it. There are a number of necromancy books out there.
The main thing I like about Ambient's work, and Green Ronin's for that matter, is that it all is put into a context. The Necromancer's Legacy is all about just that. The legacy, items, spells, creatures, templates, PrCs that have formed around a long dead(?) necromancer of great power. That context gave me all sorts of ideas for the material in the pdf version, just like Green Ronin's Secret College did. Is it all generic, sure it is. But there is just enough flavor wrapped around it to make it really meaningful.
That is the main thing to me that makes the Ambient products so good. If you have any of their others, you will have a good idea of what to expect. If you do not have any others, and like necromacer ideas and crunchy goodness, this is a good introduction to their work.
All of this is said with a reminder that I only have the pdf version. I have not had a chance (or funds) to get the print version.
Mike