Book X reads, at first, like an inventory. Viṣṇu among the Ādityas, the sun among lights, the ocean among waters, Meru among mountains, OM among utterances — dozens of them, one after another (10.21 onward). Taken as a list it is impressive, and after a while faintly tedious, the way any list of superlatives becomes.
But the chapter says what it is doing before it begins. Arjuna has asked a precise question: in which states of being are You to be held in my thought (10.17)? The answer is not here is everything. It is: I shall tell you — but by their foremost only, for of My extent there is no end (10.19). A sample. Chosen for use. Explicitly not a census.
And the use is stated at the close: whatever being bears splendour, or vigour, or power — know it to have sprung from a fragment of My splendour (10.41). That is the sentence the whole chapter has been building a habit for. Each named thing is a worked example of a move the reader is expected to make afterwards, unassisted, on things that never appear in the text: the smell of rain arriving before the rain, one particular voice on the phone, the first bar of something heard at fourteen.
That is the difference between a catalogue and a ladder. A catalogue asks to be admired for its completeness. This one admits at the outset that it cannot be complete, and hands over the method instead — which is why the chapter can end with a fragment and not an apology.
So the test of having read Book X is not whether the splendours can be recited. It is whether anything ordinary stopped you this week, and you knew what the stopping was.