In the spirit of old team-up comics, Hellboy: Masks and Monsters features the titular demon joining forces with a random assortment of characters from other books to fight supernatural monsters. The Batman/Hellboy/Starman pairing seems a bit odd, with one of comics' most famous mainstream superheroes, an independent comic character five years away from his first taste of fame with the wider public through a movie, and a character from a less well-known DC comic that bordered the line with the more experimental Vertigo offerings. The whole package works as a Hellboy book, the second half of the story works surprisingly well as a Starman book, and Batman is there in the first half to get the readers' money. The two DC characters spend practically no time together and Batman is largely superfluous even in his section, a bit of a missed opportunity to not have the three working together. A full story with Hellboy and Starman fighting Nazis and getting to know each other would also have been better, but Hellboy/Starman alone obviously would've been less marketable. Still, it fleshes out the heroic career of Jack Knight a bit before his retirement in the Starman series and Hellboy gets to punch Nazis, both of which are valuable.
The latter half of the volume is the Ghost/Hellboy special, which again fits Hellboy better than his companion. I have no idea who Ghost is beyond her series being published by Dark Horse and there's so much vague "dark past" allusion that you would assume needed experience with her comic to explain, but the character seems to not even know anything about her own backstory, so it read like a lot of wheel-spinning. Maybe it'd fit well with Ghost's own book? I can't say. Hellboy gets kicked around a lot, which is par for the course in his adventures, though here he never really gets to do anything else that's terribly interesting.
So if you like Hellboy, this is a pretty good outing, if you like Starman, he's treated with a great deal more respect than he might've under a different writer (thankfully it was his creator James Robinson penning the book), if you like Batman, there's really no meat for you, and if you like Ghost, who knows.