How many pages have I torn out?
05AB1E, 13 bytes

εD&gt;)ÅÈε€θγg}{


Try it online!

Thanks to Emigna for the heads-up on spec changes.

Explanation

εD&gt;)ÅÈε€θγg}{ – Full program.
ε             – For each book...
 D            – Push two copies of it.
  &gt;           – Increment all the elements of the second copy.
   )          – Wrap the whole stack into a list.
    ÅÈ        – Produces the lists of even natural numbers lower or equal to each element.
      ε       – For each (the modified copies of the book):
       €θ     – Get the last item of each.
         γg   – And split into chunks of equal adjacent elements.
           }  – Close the loop.
            { – Sort the resulting list.