I'll start this review with the negatives which are the same as justice league of America volume 1 & that is it only contains 2 parts of the trinity war story, this is likely to confuse readers as its the first & last parts of the story. As such I'm going too focus my review on issues 18-20.
Firstly you need to be aware this volume is not a complete story as previous volumes have been, it's a build up to the trinity war and it's follow on event FOREVER EVIL and it begins with the justice league looking to recruit new members inviting them to the watch tower. After an incident involving a certain member of the metal men, the league decides on Atom, Firestorm & Element woman.
Also a thief breaks into the bat cave and steals some kryptonite, once you get to the end of trinity war you'll have good idea who the thief is. The new members of the league take on the villain Despero who is wielding a kryptonite ring(i wounder where he got that from?), luckily for them a certain green Martian shows up and batman has a chat to wounder woman and superman about the implications of their relationship.
Even though these issues are only a build up they offer enough intrigue and mystery to keep you interested, I'm not saying you necessarily need to read them to enjoy trinity war and it's definitely not a good place to start for new readers.
As for issues 22 & 23 the trinity war I would recommend buying the justice league trinity war book as it contains the complete story in order. On the whole if your like me and enjoy Johns story telling and the little hints he leaves you of things to come then I think you'll enjoy it.
Side note another reviewer stated issue 21 the Shazam issue was included, it isn't at least not in the copy I received.