Their most recent rulebook is Imperial Armour:Aeronautica. This 113 page hardback contains every flyer, super-heavy flyer or monstrous flyer they've ever created, and provides updates rules for them, suitable for 6th edition. The book also includes a number of 'anti-air' units for players who want to combat the flyer threat head on.
On the quality side, the book does really well. each page is thick, glossy, and in full colour, with great images and big readable font. The book is on the large side, but for £26, you wouldn't want a pocket book. For £26 it may seem a lot for a book., but Forgeworld books in the past have been in the low 40's, making this seem like a good entry level price. Also, its size and quality makes it comparable to fancy photography books that are available in book stores.
In terms of content, there is at least something for every army. Some factions only get one unit, so it may not be the best purchase if you only want it for the units units for Dark Eldar, Tyranids or Necrons. However, if you play multiple armies, plan to share the book with others, or want a good read, there are certainly plenty of units to choose from, such as the Grey Knight Thunderhawk, Avenger, Flakk Trakk and the Storm Eagle (which includes the chaos variant). Some units have been added to the pile of datasheets which are allowed in normal 40k games. One of which is the Hell Talon, which used to be an apocalypse only unit. The book also has a very detailed campaign pack, with a strong focus on aerial combat.
Overall, this book is a great purchase for players who want to include flyers, but do not have them in their standard codex's. The added variety this book provides can be great, and can expand on the already interesting mechanics and immersion of 40k.
(Cover image property of Forgeworld)