The best reference on MATLAB is the new book by Higham and Higham: MATLAB Guide SIAM 2000 (click the title for more details).
The book is current through version 6 (not even released in 9/2000)
The first chapter is a tutorial and the book picks out the most useful examples and hints -- full documentation is in the much longer manual Using MATLAB. M-files and graphics are covered early in the book.