Beginner's book in signal processing with practical examples on fault detection in electrical motors
When I had a look at rotating machinery, the best reference I could find is Bob Randall's Frequency Analysis.  This was generated in conjunction with Brüel &amp; Kjær as they sold lots of nice (and expensive) vibration measuring / monitoring equipment.  A short Google search shows that B&amp;K have made the PDF of this book available for free download from their website.

The book is targeted at mechanical engineers, who (back in the day) were never exposed to the Fourier transform.