This book, frankly, exhausted me. In terms of the information presented, it's very good; you're not going to find a better English-language one-volume history of the entire period in which the Two Koreas have existed. She places the war itself in the proper context of the long-view of Korea's divided history, and gives much more focus to Korean impetus and sources than you'd usually get in the American-centric histories that dominate the reading lists here. That said, though; it's just a bit of a slog to _read_. The level of detail occasionally approaches overwhelming, and Jager doesn't provide the best narrative flow in which to situate, comprehend, and retain that detail.
To start with, if you don't already have a good grounding in the war itself, this book is not going to give it to you. For example, there's much more in this book about the rebellion of POW's at Koje-Do than there is regarding, say, the Battle of the Chosin Reservoir. The latter is covered, but not to the depth of events elsewhere. And that's FINE; I believe her whole point is to restore the agency the Koreans usually lose in English-language histories of the conflict, and therefore she chooses to focus more on events that haven't been covered well before. Arguably, in terms of KOREAN history, the rebellion at Koje-Do mattered more than the retreat from Chosin. Just know that going in, and read something like Halberstam's "The Coldest Winter" to get grounded on the war itself.
More problematically, even though she does present a lot of detail overall on the war era, it's provided in a weirdly staggered way that makes it hard to follow the actual chronological flow of events. Or at least it was for me.
That said, the book also succeeds on a lot of fronts, particularly on the level of new information presented regarding the domestic South Korean front and the entirety of the Communist side. While again obviously focused on presenting the Koreans as their own actors with their own methods, she does not shrink away from showing that the Rhees and Il Sungs of the time had their actions strongly circumscribed by what their ideological big brothers would allow. For all of the loud trumpeting of the North Korean autarkic ideal of "juche" they have subjected everyone to over the decades, the historical record makes it clear that, at almost no times throughout its history, has North Korea been able to decide entirely on its own course. Rather, the South Koreans have had more success making their American patrons react to their own actions than the North Koreans ever did at getting the Chinese or the Soviet/Russians to theirs.
Jager also does well in presenting an honest picture of the economic race of each Korea in the post-war era; it sounds surprising to consider that North Korea was actually probably ahead of the South Koreans economically until the mid-1960's, even after being bombed mostly flat by the American Air Force in the war, but the South Korean leadership was that bad at caring about or knowing how to improve the lot of the average citizen for quite some time.
I enjoyed Jager most when she discusses how the poor economic performance of early South Korean governments led to civilian dissatisfaction with western-style democracy and, instead, resulted in support for the Communist cause that seemed to be doing a better job of raising the standard of its people at the time. This carries into the reaction and repression that stripped away rights from the South, but also provided the stability necessary for genuine economic growth to finally occur. In turn, that increasing wealth empowered the people to where they were able to force a return of democracy. Meanwhile, throughout, the North simply became increasingly poor, isolated, and unstable.
Overall, I can give Brothers at War a qualified recommendation; if you're well-versed in the war itself that is the dominating event of this entire era, and can deal with the occasionally confusing and meandering nature of the narrative, this book will fill in the periods before and following the war better than anything else you're going to find in English today. If not, I'd recommend reading some dedicated histories of the war itself first, and/or waiting to see if anybody else decides to try and replicate Jager's work here in the next few years in a clearer, more cohesive fashion.