Now's when I crack out my Alaadin voice and start singing, "I can show you the world/ shining, shimmering, splendid..." (Oh, wait, I can't sing. Right.)
Now, keep in mind that I have only done the most basic knotwork techniques, so I only searched for basic tutorials...
I would like to introduce you to the website that made Celtic Knotwork "click" for me. (My brain clicks. No clicking = no understanding. But once something has clicked, I won't forget it for a long, long time.)
Here is her tutorial page. The grids with the big and little dots are what made it click for me. You can print the grids for free, here. I recommend reading (and trying!) the basic knot and how to modify the basic knot.
I found a random blog that made the whole idea of "walls" click. That Jolie Girl has a tutorial here, and some very helpful notes here. And, wow, I just found this, which is very helpful, also.
This free PDF tutorial by Reed Mihaloew may be technical and in-depth, but just from a cursory glance it looks like one of the best I've seen so far. I plan to use this tutorial to teach myself some more complex knots.
You know, if I had the experience, I would totally make some tutorials of my own. I honestly don't think there is much out there that is very helpful.
(Although I imagine that books dedicated to the subject are very illuminating-- haha, I made a joke-- but our library doesn't have any.)