If you want start with multi-touch development you have to decide what topics you are interested.
Some prefer only the interaction design, others trying to build their own soft-/hardware from the ground. Whatever you like the nuigroup forum is THE starting point on the net. There are a lot of information spread all around the forum. Without the help of so many community members I never would have achieved the things i develop.

To get the historical background and wider context on this field you should go to the site of Bill Buxton. He is a Principal Researcher at Microsoft and if somebody has a credibility on this topic then he. There is a very interesting talk from the CES 2010 with him about NUI: CES 2010: NUI with Bill Buxton