It's pronounced "Kelts".
There is a lot of great information about the Celts available on the Web. My goal is not to reproduce it here or maintain an exhaustive list of links. I'll provide a few pointers to get you started, and add some content where I feel I can contribute something useful.
If you're interested in learning about Celtic history or culture, you should check out my list of references, which is a bibliography of online and offline resources.
I became interested in the history and culture of the Celts quite accidentally. I saw an example of Celtic knotwork, and (as a compulsive doodler) I decided to learn how to draw the knots myself. I picked up a book by Iain Bain (which is cited in the bibliography) and learned how to draw them.
I've put together some instructional pages so that you can draw Celtic knotwork, too.
Many readers send me email with questions about the meaning of Celtic knots. These questions prompted me to do some research into the origin and meaning of Celtic knotwork.
The most impressive knotwork project I've undertaken is the Celtic Cross, which I completed on St. Patrick's Day 1997. This design was inspired by a gravestone I saw in New Hampshire. The Celtic Cross reflects the union of the Celtic and Christian cultures, which have become deeply intertwined. For more info on this aspect of the Celts, see the Celtic Christianity Web site.
It's not exactly knotwork, but I copied an interlaced pattern that I saw on a chanfron. The chanfron page also includes a few notes about how I created the image, in case you'd like to see an example of how to construct or copy an interlaced design.