For the time being let me only mention that the pages by Günter Janska were very useful for us and, in particular, the link to Humberto. Humberto is a teacher in Havanna who also works as a tourist guide. He showed us Havanna and arranged various other things for us (casa particulares, delicious dinners, Viazul reservations, etc.). The Havanna city tours with him were really great! He is very open minded and you can ask him pretty much everything to get some insights which you are unlikely to find in your travel guide. Moreover, he is a very kind person and we quickly became friends.
So, if you are planning an individual trip to Cuba and quality is more important to you than getting a casa particular for a few dollars less, contact Humberto!
PS: And learn Spanish! Humberto speaks English fluently, but most Cubans don't.