“Spinoza's point is they are of identical reality. The a priori definition of the modes insentience and sentience in the attribute of thought are of the same reality. Both definitions are true of reality. It isn't the case we have one otherworldly plane with just the definition of sentience and another with just the definition of non-sentience.
Much like all the books on my shelf are entirely different objects, yet all of them are identical in being of my shelf.” — Dependency on another, &, being identical in reality aren’t the same thing (offspring are dependent on progenitors, not literally identical with them in reality), an important point that’s been repeated a few times now. Sure, Spinoza holds that both are dependent on one, single thing (indistinguishable from or identical with itself), but not that the modes of thought & the modes of extension are identical or of an identical reality; in fact, contrary to such a position, he maintains that both can only be conceived without & independently of each other.
Moreover, the relationship between the two can be, in fact, likened to one worldly plane in relation to another, sort of like parallel universes, in as much as he asserts a parallelism between the reality of states of thought & that of bodies or body, hence, he writes,
“The order and connection of ideas is the same as the order and connection of things” (“The Ethics”, Part. 2, Prop. VII); in other words, the series, order or connection of states of thought/ideas correspond or parallel the series, order or connection of states of bodies/bodily things (despite the fact of each of them only being conceivable independently of the other), according to him.
Also, your bookshelf comparison isn’t admissible in the way that you want to use it here, for a bookshelf is an empirical object that’s sensibly perceivable apart from, &, or, relatively positioned with, the book or books which it holds, while this isn’t the case for the modifications of “substance”; yet, nonetheless, simply note that not only is each book separable from the bookshelf, but also from one another, further proving how distinguishable things can never be identical, i.e., can never be thought of as being one & the same, & therefore necessitate a relation between diverse things as their justification or explanation.
“This is mistaken. For Spinoza, substance isn't predicated over other objects. It's self-defined. There is a thing, the absolute infinite of substance. It is not merely a subjective categorial term. Nor is it a mode of extension (e.g. an empirical state, an instance of some thinking the concept, etc.). Nor is it a mode of thought (e.g. an a priori definition of a mode of thought, like the meaning of sentient or non-seinteint).
It is real outside anyone existing conception and judgement. It is, in the the terms you are using, a thing-itself. Substance is there whether or not anyone thinks about it.” — The referent of the term “substance” or “thing” isn’t a categorical qualification, the point is that the determination of a given thing being in-itself, or, in-another, is; this fact of being either is never self-evident in any bare perception or experience, & so only ever categorically understood. Moreover, “substance” is predicated over “objects”, in as much as it’s acknowledged to not be restricted to or entirely immanent in any particular states of objectification, for it’s beyond & transcends such limitations & constraints, though it’s the condition of them.
“I wasn't suggesting they were contingent states. For that, I would have to be referring to a contingent entity which was sentient or non-sentient. I was talking about the a priori (so definitely not contingent) and how as a mode of thought, it could not justify itself. The point being explaining a priori concepts is more complex than just asserting their necessity (though asserting their necessity is a description enough to describe that feature).” — In the way that you’re speaking of it, to “have” something, or to possess something, particularly sentience or insentience, is or would be a contingent state, though; in as much as possessor & possession are never necessarily related but only contingently (like your books & bookshelf, there’s no necessary relation between them [those books weren’t always & don’t have to be there]), & so aren’t absolutely united (as mode & substance are). Yet, to be clear, it’s not solely about asserting the want of logical necessity, when objecting to Spinoza’s conception of “substance” & “mode”, but also that of a qualitative essentiality which is lacking in his very formation of the conception itself.