Behind The Masc is a series of games and gaming materials orbiting around themes of masculinity.
It's got content for 5e, Monsterhearts, and Apocalypse World, it's got two text adventure games, and it also includes designer notes from all contributors, giving some cool insight into the creation process and the way people connect to their material.
The whole book is 39 pages, with extremely readable layout and a few cool illustrations.
I think the first expectation with a work like this is that it will be ultimately scholarly and analytic, but this isn't. Masc is intuitive and playable, and the 5e, Monsterhearts, and Apocalypse World content doesn't feel like it clashes with any of those games' milieus unless you specifically set out to exclude it.
The writing throughout Masc is also really strong, and while masculinity is the central theme, it certainly isn't the only thing discussed.
Probably my favorite piece in the book is the Monsterhearts Minotaur skin, which deals primarily with the tension between getting involved, helping people, and risking yourself, or staying isolated and safe.
Overall, this is a cool project, and if you're looking for material for any of the named games that deals with, deconstructs, and re-constructs masculinity, I'd say look here first.