I've finished the TMP book through the end of production/postproduction. I still have another third to go before finishing it. I will say that Cushman's access to memos in the Roddenberry Archives has been very illuminating.
There's a nice detailed memo from Roddenberry to Wise regarding the Director's Cut as it stood in April 1979. Roddenberry's comments are well thought out, service the story and the film's structure. Many of his suggestions were implemented, such as adding more intercutting of characters (generally a take was held on a character as long as they were talking rather than cut away to a reaction while they were talking).
Something Roddenberry lobbied hard for was the reintroduction of the Captain's Log entries in the film. Wise didn't like the idea of Log entries but relented knowing that this could provide a way to clarify that Spock's arrival was pivotal in solving the anti-matter engine imbalance. The log entries that made the final cut were written by Roddenberry.
Cushman doesn't let Roddenberry off the hook for large portions of the book but these post-production insights were very interesting to read.
I really liked the log entries in TMP. Each of the other movies didn't have them feature as prominently and mid-story like TMP did.
TWOK features 3 (Saavik in the simulator, Chekov on approach to CA5, Kirk at the end)
TSFS features 1 (film opening- Kirk)
TVH features 1 (Kirk on Vulcan)
TFF features none (an aborted log by Kirk doesn't really count)
TUC features 2 (Kirk upon leaving spacedock, Kirk at the very end)
GEN features 2 (Picard prior to Stellar Cartography scene, Picard at the end)
FC features 1 (Picard- just after his nightmare)
INS features none
NEM features 1 (Picard- in orbit of Romulus)