Barnes & Noble, often the best price but not always
Amazon.com, easier to find and often the price to beat
OpAmp Books, the oldest and best, with the best price for Gelb
Gelb Example 1-1 using vector notation, showing how much easier things are done using vector notation as opposed to scalar equations.
Z Transforms, a short tutorial
The Matrix Inversion Lemma, also a short tutorial in simple matrix algebraic manipulation.
Determinants, a fairly long write-up showing many things. The vector triple-product is a determinant, for example.