I just finished 君を愛したひとりの僕へ | L29, and I honestly don’t know how I feel about it… it was still a good book, but the tone was so different from 僕が愛したすべての君へ | L29. While I felt happy reading 僕が愛したすべての君へ, I just felt depressed reading much of 君を愛したひとりの僕へ. I don’t mind the occasional sad movie, but reading a sad book is different since it’s over several days instead of a couple hours.
I rechecked the description of both books on Amazon to see if there was an intended order, and while I still don’t think there was, I think reading them opposite of how I read them might have been a better experience. Both could probably be enjoyed individually and they can probably be enjoyed in either order as well. That’s because they both reference and (loosely) spoil events from the other, and you’re likely to have some “ah ha” moments with whichever one you read second, but in neither case are the events of one a requirement to understand or enjoy the other. However, based on certain story points that I won’t spoil (or if you just prefer a happier tone like me), it’s probably best to read 僕が愛したすべての君へ second.