In the last few weeks, I've picked up:
- Mixing in Sonar (X1),
- X3 Explained,
- X3 New Features, and
- Complete Sonar X2.
I've only watched a few of the last two so far, but the others I've watched from start to finish. All (from what I've seen so far) are crammed full of helpful information.
And I bought these after buying, "The Power in Cakewalk Sonar" (X2) which includes a DVD containing pretty much everything covered in the book and is, IMHO, as valuable as the book itself.
And despite the fact that there's a lot of overlap, every one of these sources comes at the subject from a slightly different angle and so they're all helpful in their own ways.
I also picked up "Mixing Secrets for the Small Studio" on a recommendation from (I think it was) Bitflipper, another member of this forum, and I'm happily wading through that as well.
Almost all the information about X2 is applicable to X3 and what little that isn't becomes apparent quite quickly.
Hope this helps.