Textbooks

• Hansen, Arthur, "Similarity Analysis of Boundary Value Problems in Engineering," Prentice Hall, 1965
  
  Hansen gives an exceptionally practical perspective of similarity solutions in engineering.  He presents several methods and examples in each.  It's written in such a way that the mathematical theory is preserved but the focus is on the application.  This is the single best source to learn similarity solutions I've read.
• Haberman, Richard, "Applied Partial Differential Equations," Prentice Hall, 2003
  
  This book is a must have to learn PDEs.  It is readable, practical, and includeds commentary on the many pitfalls encountered when solving PDEs.  I highly recommend this book to anyone learning PDEs.  It has many physical examples it nicely relates the mathematical significance to.