There are 2 great books out there at the moment....
XSI Illuminated : Foundation
Softimage|XSI Character Animation f/x & Design
If I were you, I'd pick up both. They are avaliable on Amazon.com
If it's a video that you want... (warning: shameless plug ahead)
I've made a video on SoftimageXSI called "SoftimageXSI: Mastering Materials". You can find more info on it here: http://www.edharriss.com/Tape
While it does not go over ALL the basics, it does go over a lot of them. Mostly the ones that deal with textures and materials. But to support the projects some areas outside of textures and materials are explained. While the manuals are good, if you want to learn and learn fast... Nothing beats a live demonstration. I made SoftimageXSI: Mastering Materials for beginner and intermediate users. But It's also useful for users of every range that are transitioning from Softimage 3D to SoftimageXSI. Every step is explained in detail to ensure that the beginning and intermediate users can learn as quickly as possible and the advanced users can follow along with ease. When most dialog boxes and pulldown menus get used, I zoom in on the appropriate portions of the interface so that they can be clearly seen and the user can better understand the current lesson.
There is also another tape that covers the basics of XSI. But it's only in Japaneese. It's called SOFTIMAGE | XSI Tour and can be found here: http://www.gogo3d.com/xsi\_tour.html
Hope this helps.
Ed Harriss, http://www.EdHarriss.com
New Video! "SoftimageXSI: Mastering Materials"
Now Shipping - see Examples at EdHarriss.com