I think the Harmony Word of Mouth thread that was linked earlier would be helpful to you if you haven't looked at it already, but since you asked about what makes it a bad book, the role that the show has in the first act is to get audiences invested in it. A really good book will set up the story well so that audiences want to come back for the second act, create fleshed out characters that people care about or are interested in, and present the musical numbers in a way that audiences can enjoy and also move the story forwards. A lot of the time the book won't do all of these things, but will do it enough that the show is still enjoyable and audiences are convinced they want to see another half of the show.
Harmony I think falls into the trap of trying to do too many things with the first act. While Rabbi is the main character since older Rabbi is the narrator, I think it's still hard to care about him as a character unless you're personally invested in Chip Zien (which to be fair, many of us are). The show starts off like many jukebox musicals, with getting the band together and trying to chart their rise to success, but we don't ever see them perform a full number together until about 2/3 of the way through, so it's hard to be convinced that they should be successful outside of just taking their word for it. And while there's talk of the revolution, it's done in a bit of a cliche way and leans pretty hard into the "we as an audience know what's going to happen and it's not going to end well, but the characters are still optimistic" aspect, which I think works for some people but has come to be an overused trope imo.
So at intermission we know there's a singing group that managed to get to success (but they've still only performed one real number for us to judge that success on); there's not really a single character that stands out for us to be invested in (Rabbi fills this role to a certain extent, and Chip Zien is giving it his all, but by this point he's still just not that interesting. The main thing we know is he runs away from things.); there's revolution brewing, but we can kind of predict how it'll go; and there's a wedding, but like much of the other parts of the show, the romance is more told to the audience rather than something that feels organic (a version of telling rather than showing), so it really feels like it was given more weight than it deserved (I don't know if we really needed the whole wedding scene to be shown).
I say this as someone who does like the show and really enjoyed it off-Broadway because I think it has a great story, but it's very hard to tell what that story is supposed to be after the first act. The second act is written a lot better, where all of these different threads manage to come together--the political tensions give way to the kind of performances that the group does, and Ruth's role in the revolution spurs on some of the decisions they make (as well as some of the tensions within the group), and we also see the show crystallize as a portrayal of Rabbi's feelings in looking back on that time. But the problem is that after the first act, we don't really see the full potential of what the story is going to be since it's really just moving a lot of pieces into places so that the second act can work. And while this isn't inherently a bad thing, it's just not as enjoyable and makes it confusing to describe what the show is actually about so it feels a bit like "what's the point".
