This question involves minor spoilers involving Lasciel, read with caution.
There is a section in one of the books where,
Harry has been kidnapped, locked to a wall with spiked anti-wizard manacles, and is being auctioned off to the highest bidder. He manages to convince Lasciel to help him without taking up the coin. To do so she tells him how to dislocate his thumbs and allows him to see in the dark using her ability to use a perfect version of Harry's memory.
In which book and chapter did this happen?