Hey
The best resource for understanding Matlab basics is the Matlab help itself.
Just go to the help for the toolbox you want to use.
Learning Matlab will give you little help in learning DSP.Matlab is just a tool to code DSP Procedures and see the result.
I agree that MATLAB help is very helpful in learning matlab. If you want to start from ver basics go go "getting started with matlab 7" by Rudra Pratap