Bug Notes
This was disabled in version of 1.02. However, it was re-instated in version 1.05 of the patch and is now working correctly.
Other Notes
Aside from the summons in your spellbook this feat also works with the Negative Plane Avatar granted by death domain, or any creature summoned from an item (such as the Djinn from Storm of Zehir).
The feat does not work on Animal Companions.