Learning Ajax, Json, with php and database manipulation.

I've been struggling today and last week to find some good tutorials or a well recommended book for getting the ball rolling in my quest to learn Ajax. I figure I might as well try to start off working with Json instead of wasting time with parsing string data. So far here are the links I've found useful:

Quick run down on JSON:
Understanding JSON: the 3 minute lesson
JSON on Wikipedia

Excellent Starter tutorial using Ajax (and ASP, but can be easily modified to a php date() function)
W3schools Ajax Example

More Advanced and In Depth Series on Ajax covering numerous topics, which I breezed through the first 3. A much longer read, but contains some really useful details:
Mastering Ajax: 11 Part Series from IBM developerWorks

After you do the W3 Schools Tutorial, I suggest moving on to this one, which uses Ajax, Json, Php, and Database manipulation. Heads up - I had to remove the $$link variable variables from the database functions in order for it to work.
Json Ajax Webchat Tutorial

Good luck, and if you know of some additional resources I would appreciate the links.