"The USS Enterprise NCC-1701 received some temporary modifications while under the command of Captain Christopher Pike..."
The idea that they changed after "The Cage" for Disco (making it bigger and totally changing the proportions) then changed it back for TOS proper is one of the stupidest things I've read in a Star Trek book...
...I feel sorry for their writers. They're trying.
Exactly. It wasn't their idea to change the design and the size of the TOS Enterprise, but they got the job of trying to explain away the discrepancy, since the book makes it clear that they're the same ship from the same universe even though it's rather an insult to the reader's intelligence that this is the case.

Ok, but seriously it’s a publishing error to depict the design lineage like this since DSC would’ve restyled all the other
Enterprises to match.
But it wasn't the book authors' job to speculate how future Enterprises would look like with a 'visual reboot' (such a silly term, IMHO). It was their job to shoehorn in the DSC Enterprise into the already established Enterprise lineage from everything that came before DSC (other than the alternate universe of the Kelvin timeline) and then come up with some excuse to explain the changes. And as
@King Daniel Beyond pointed out, their explanation was as about as inventive as Kurtzman's explanation about how DSC fits into the prime Star Trek canon: classify everything, send the ship 1,000 years into the future, and sweep the last two years under the rug and start over again
Except the DSC version is just as canon as the rest
Yes, it's absolutely canon to the Star Trek universe. Except I'm not really buying the book authors' explanation (again, not their fault) that the Enterprise went from how we saw it in "The Menagerie" to how we saw it in DSC, to how we saw it in "Where No Man Has Gone Before." I think the current producers of DSC have realized that making a series that takes place ten years before TOS but looking nothing like TOS was a mistake, which is why we're getting ST:Picard (among other reasons.) 20 years from now, people aren't going to fondly remember the 'look' of DSC when thinking of people like Kirk, Spock, McCoy, Scotty, the Enterprise, the Klingons, etc. They're going to fondly remember TOS. This is of course just my opinion, but hopefully I'll still be around 20 years from now to see if I was proven right.