Poschengband: Book of Shadows?

[Brouhaha]

Anybody know where my skillmaster can pick up the 4th burglary spellbook?

[murphy]

The fourth spellbooks are rare for all realms. I think your best bet is killing deep level uniques

[GenericPseudonym]

For classes other than Skillmaster you get guaranteed drops of your third and fourth spellbooks from Smaug and Null the Living Void, respectively. They also drop pretty frequently as rewards from 'quest' uniques in Angband.

Unfortunately Skillmaster will just have to rely on vaults and random drops. This might have changed at some point, but last time I played, they always got Sorcery books regardless of which realms they knew.