With books what does “pp” mean, e.g. “42pp A4”?

Booklets and brochures can only be made up of pages that are a multiple of 4 when saddle stitched and multiples of 2 when perfect/wiro bound. This is because of the way they are constructed, so if you imagine a sheet of A4 paper folded in half to make an A5 booklet, you are left with 4 printed pages (pp). In order to add more pages you need to print and fold another sheet, therefore you add another 4 pages each time. Be sure to detail whether the cover is included in your overall page count or additional to it.