I'm undergraduate and I'm looking for a text about Small Oscillations in which matrices are used. Could you suggest me a book or a PDF file?
I think you should try these books:
1) David Tong: Lectures on Classical Dynamics (http://www.damtp.cam.ac.uk/user/tong/dynamics.htm)
2) V. I. Arnold, Mathematical Methods of Classical Mechanics
3) L. Landau an E. Lifshitz, Mechanics
The material is standard, so you can probably learn it equally well from many sources, some of which are referenced in the link mentioned in the (edited) question.