What is the extraNonce?

This question helps identify the various components of the getwork “data” field, but I’m curious where the extraNonce comes from. My hunch is it’s the first four bytes of the second half of the data, because I believe this is still part of the Merkle root (and yet, being in the second half of the data, it won’t be part of the midstate). Is this correct? And if so, how does one increment the extraNonce, and what does it mean to do so?

Example getwork data:

Is the extraNonce (in little-endian, I know), 55b5b8f6?

Bitcoin URL scheme

What is the URL scheme that is used by Bitcoin to copy transaction information directly into the client? Which versions of Bitcoin clients support it?

Any books about Bitcoin?

Thanks for any link/author+title. I prefer max. 2 years to the past if possible. If you know about some book that will be published in the near future, you can mention it too.