Weird Adventures is a sourcebook for a game setting that is a mixture of traditional fantasy elements and a mad infusion of weird pulp fiction (mostly) set in a strangely familiar nation and city.
The sheer density of setting texture and detail alone makes this sourcebook a must-buy, but it's tied together by a weird conglomeration of almost-recognizable elements taken from history, period culture, myth, movies, fiction, comics, and pulp novel arcana that works as a setting. There is some lacuna left for the tastes of the DM -- but those spaces are easily filled by nabbing from both traditional fantasy and 1920s Americana.
It is a bit light on the game mechanics for a sourcebook, but the new monsters have stats that can be extrapolated to any D&D ruleset, and the DM is open to establishing how prevalent and powerful guns and transport might be in his/her campaign. I do recommend allowing yourself to be inspired by both magic and mad science in answering these questions, as the setting seems to excel at keeping players and GMs on their mental toes in anticipating secrets and twists to the adventure hooks. I'd love to give examples, but to do justice I'd have to give an entire entry away!
The art truly evokes both the feeling of the source material and the conceit of being a travel guide of sorts into this strange realm, and -- while I only have the PDF version -- I think that the printed copy will make a handsome, conversation-starting addition to any gaming collection.
Just make sure your friends don't borrow it without you knowing!
I'm think that an appropriate expansion would be various rulesets expansions for the appropriate retro-clone, but that's another post.