The Masks team continue to produce excellent material.
Where the Halcyon City sourcebook was a wide-ranging slice of life in the Masks universe, Secrets of A.E.G.I.S. is more tightly focussed, concentrating as it does on a single organisation.
This has good and bad points. The upside is a thorough, tightly aimed look at the interactions between AEGIS and the game. The books opens with a number of reports on organisations monitored by AEGIS agents and their interactions; each of these provides ample fodder for a campaign in a variety of different styles. They cover plots involving crime bosses, supervillains, local politics and superhero activists - something for every GM to work with. Most of this chapter will be useful material will be useful even if the GM doesn't plan an AEGIS-centred campaign.
A chapter is also devoted to discussing how to centre a campaign on players as AEGIS agents, with some good general rules material for handling organisational agendas.
On the down side, the language is occassionally coded in the style of over-formal or jargon-laded AEGIS internal reports; it lends a bit of extra atmosphere but sometimes lacks the breezily readable tone of the Masks rulebook or Halcyon supplement. This is most notable in the opening chapter, so it took me a little while to get started... but the book rewarded the effort.
The new playbooks are clear and provides interesting variation from the base set, without being over-tightly tied to AEGIS as an organisation - it would be trivial to substitute some other organisation in the Soldier playbook if AEGIS is not a feature of the group's play. They'll be useful in any campaign.
The highlight of the book for me was the "Most Wanted" section, a detailed examination of some existing antagonists. A few well-chosen villains from the deck of villainy are given a great deal of personality and scope with in-depth discussion of their history and objectives. Any of these writeups left me with scenes and campaign ideas in abundance.
Disclosure: I'm a kickstarter backer of Masks, so my copy of this book was paid for in advance. But reading it has made me glad I did. A rewarding read full of usable ideas.