The funny thing is, I grew up watching a copy of TWOK that had been taped off of a Fox showing, and it includes the scene about Peter being Scott's nephew. I honestly never thought about that scene not being in other cuts of the movie.
The novelizations of TMP, TFF, and TUC all have a lot of extra elements that I think would look good on film. TFF offers background on the three ambassadors at Nimbus and Sybok, as well as giving tension to the Klingon crew. Klaa won a promotion to captain through his skill as a gunner, and the firing rig he uses in the movie is a special modification. Some of his crew think he's little more than a political favorite.
The TUC novelization has a much better version of the "book scene" where the Enterprise is talking with the Klingon outpost. Chekov states in the film that the UT can't be used because it would be "recognized," whatever that means, but in the novel the conspirators disabled the UT. It's also stated that the Enterprise's photon torpedo system generates a small neutrino surge when it's active, giving context to Chekov's remark that only a ship could generate a field "that large." This gives the reader the question of whether the conspirators are actually able to access the Enterprise's launchers when the attack on the Gorkon occurs.