If you start reading a spellbook and then get interrupted by a monster while reading, the game asks whether you want to keep reading or not, as intended. However, if you answer "no" to this prompt, you stop reading, yet learn the spell in question immediately (or fail learning the spell and suffer possible consequences of failed reading).

Edit:
This seems to affect other actions too, not only reading. For example, eating can be interrupted, yet is still finished immediately if answered "no" to the prompt.