Alright here's my take, as a teen birder:
I honestly just don't see the value here. I have never heard of "minibooks" before this, and I will likely never buy one. I see no problem with just getting a paper/hardcover and plopping down somewhere to read for a few hours. I don't need to feel like I'm swiping through an Instagram feed to read.
I rarely take a field guide into the birding with me as is, and really haven't since I started birding. I never used one of those "pocket-sized field guides" just because they did not have the detail that I could get within a Sibley or NatGeo. I'm just trying to imagine something with even smaller font/images. When I started birding in mid-2015, my NatGeo proved to be a very useful guide, as I could see similar species and compare the two on one page. It seems to me that in a "minibook" that this would not be possible, just due to the tiny page size. With the smaller page size too, I think that many of the illustrations would lose much detail, with many of the key identifying features being harder to see/not visible at all.
I honestly don't see the problem with a phone either. Granted, they do have battery life (which I just pack a good portable charger for), but the cellular issue can be fixed fairly easily by downloading packs and whatnot for offline usage. The advantage about the field guides on a phone too (I only have Merlin), is that they offer birds in multiple natural positions, as well as a variety of sounds.
To me, there really would be no need for a minibook field guide. I'd much rather save my money for something I'd actually use.
PS: The demo image technically violates forum rules, the words second from top left and somewhere in the middle on the bottom page seem to be a bit expletive.