Okay. So another CC modder has released some MORE magazines that can be read by regular Sims. They seem to really be THE item of CC to have right now.
Thanks to this I have managed to solve the issue of the books stretching when being read. The ones that stretch are using tuning that classes them as "book". The ones that don't stretch are using tuning that classes them as "bookThin."
The only issue that remains now though it the Sims' hand pose when holding a closed magazine as that still isn't right with any of the CC magazines I have found.
You can take a look at the latest magazines I found here> http://bootstrap.modthesims.info/download.php?t=577301
They are the last item on the Files tab list.
ETA: I just discovered some of the base game skill books that are thin also suffer from the wrong holding position. The level 1 fishing skill book, for example, is held correctly by Sims but the "How to be a socialite" Charisma skill book isn't.
ETA2: Mystery SOLVED! (I think)
The tuning for the thin books that are held incorrectly basically have the wrong carryObject value set. I realised changing "book" to "bookThin" under object TYPE would stop the stretching but didn't pick up on the fact that in the line above, the carryObject sets what animation to use and that was also set to "book" instead of "bookThin". I just made an override of the socialite book tuning to make the required change and my Sims now hold the book exactly as they should.