To quote my first contribution to the wiki:

Note that Post-It adhesive leaves a residue and is also acidic. If your Moleskine is for daily use, you may not care, but if you are using it as a diary or long term sketchbook, you want to avoid the use of Post-It or similar temporary adhesive products. Reference: http://orpheus.ucsd.edu/preservation/postits.html

Obviously, for most books it doesn’t matter whatsoever. But it is worth knowing for those few books that are valuable or otherwise long term items. I use post its all the time… but I now avoid putting any on or in my collection of golden age pulp magazines and other collectables.