Structurally dubious on the whole, but the back sheet of plywood is probably doing a large part of holding it together, evidently adequately. I do see top brackets to keep it from tipping over.
I doubt that there's a steel support between each set of shelves, but that would be a way to tie it together better, if wanted.
Functionally, it's the all-wooden version of plank and cinderblock (or brick) bookcases. A functional limitation is that the shelves are not adjustable at all, but if you are fine with that, you could certainly replicate this. I would expect that the shelf is drilled and screwed (or nailed) into the support below it, and the hidden side of the support is pocket-screwed (or toe-nailed) into the shelf below it (staggered so they don't conflict) and then the plywood back is attached to the shelf assembly to stiffen it up and unify it.
The bottom-most shelf would presumably be screwed up into the support above, rather than the support being pocket-screwed into it, and the feet are offset so they can be screwed down into from the bottom shelf.
There may be an additional sctructural detail lurking in the back corners, mostly hidden by books.
I think this shows the foot/leg board extending up inside the back corner of the shelves, which would provide a good solid hunk of vertical timber to attach all those separate end bits to, and make the structure more solid than I had at first guessed. But it's not showing in the third picture, so it may not extend all that far up. Or the different sets of shelves may be built slightly differently.