I am looking for an introduction (textbook or paper, or if someone wouldn't mind writing a memo on it) to canonical quantum gravity, specifically the concept of minisuperspaces. I have been reading the paper of Wheeler (in Phys Review) but he doesn't speak about minisuperspaces, maybe they were not used/invented at the time.
Well, there's Bojowald's Canonical Gravity and Applications.
For mini-superspace, hmm...Thiemann's book might be good. I know Kiefer's Quantum Gravity discusses it a bit, and contains good references therein.
For midi-superspace models, there's a review article: Barbero and Villaseñor's Quantization of Midisuperspace Models.
There's also Misner's "Minisuperspace" (in JR Klauder, ed., Magic Without Magic: John Archibald Wheeler. A Collection of Essays in Honor of his Sixtieth Birthday, pp. 441–473).
Ashtekar et al.'s "Minisuperspaces: Observables and Quantization" arXiv:gr-qc/9302027.
I think these should get you a good overview...
How about http://www.amazon.com/Canonical-Relativity-Cambridge-Monographs-Mathematical/dp/0521741874, Modern Canonical Quantum General Relativity (Cambridge Monographs on Mathematical Physics), Thiemann?