It has a few new rules for new aircraft - Marauder Colossus and Vigilant, Ork Bommer, Remora Drone Fighter, DX-13 Aerial Mine, Harbinger. It also has some aircraft upgrade rules - things like ejector seats, star engines, etc, covering 1.5 pages.
There are also full squadron lists including the original aircraft and the new along with adjusted points (for instance, Eldar Vampires went up from 24pts to 32pts each). And then there are new datasheets for some existing aircraft - these have different weapon options than in the first book, such as the Eldar Phoenix with starcannons; there are 16 datasheets covering the all new aircraft and new variants.
There is a campaign play system, and some new scenarios. And also some additional rules for handling ground installations. Finally there is a battle report (this starts on page 103), and then some discussions of tactics to use in the game (which starts on page 122), finished off with an
FAQ page and a designers notes section.
In all there are 25 pages of rules, datasheets, and scenarios, plus a few more for the tactica and the
FAQ. It might not sound a lot, but it seems like a higher proportion than the first book did.