It is indeed a "love story", and not only a metaphorical one -- which will likely make it much more accessible to some readers. It is also the true story of a philosopher falling into "book heaven" in the White Mountains in the form of the abandoned library of William Ernest Hocking. Having gotten engaged on a rock in a mountain stream in the White Mountains, there was a an emotional connection for me.
While I am an inveterate page tabber, there were only two in this book -- it turned out to be more recreational than serious.
My summary of the philosophy of James is that -- "We the Pragmatists believe it is no longer possible to accept the transcendent after Darwin, but find the fact of a meaningless life based on the random effects of materials sloshing around completely undirected to be existentially so depressing as to to make suicide the only viable option. However, that prospect doesn't seen so grand either, so we have decided to muddle on -- perhaps beauty, perhaps love, perhaps mere stiff upper lip determination will suffice for us to carry on to the inevitable annihilation of death. We live in the hope that something will turn up!"
Kaag seems to have gone somewhat on the path of Charles Sanders Peirce, likely with "love" being the breakthrough vs a "religious experience".
"and he (Pierce) never tells us what happened in his religious experience at St Thomas's or exactly what his communion with the Absolute was like. All he tells us is that he was radically, irreversibly changed: "I have never been a mystic before, but now I am".
On and around page 226, "it wasn't some deus ex machina that would save me from my situation".
"deus ex machina", "God from the machine" a term typically associated with film or writing, where "all of a sudden", something completely unexpected shows up and saves the day.
Shortly after he quotes Plato "Philosophy begins in wonder. And, at the end, when philosophical thought has done it's best, the wonder remains".
A less brilliant thinker than your typical philosopher might just substitute God for "wonder", and say something like "The fear [knowledge, respect. ...] of the LORD is the beginning of wisdom" (Proverbs 9:10)
A more current thinker might say that after Heisenberg, CRISPR, and other genetic and physics discoveries, Darwin is dead, so we are back to God and "wonder".
Naturally, modern man is REALLY driven to reject the God hypothesis, no longer because he not only "has no need for it" (Laplace), he can't possibly allow it, because it would force him to reconsider his worship of self and pleasure, and THAT is something he simply can't countenance!
The book is an entertaining read, and Kaag has an easy style. Hey, "William James" and "Pragmatism" sound more impressive than "a shallow cotton candy romance novel", so many moderns will love it. It also makes adultery into a courageous, morally imperative life growth event -- so there is that!