Walter A. Strauss's Partial Differential Equations: An Introduction is a classic PDE textbook for the undergraduate students. While Folland's Introduction to Partial Differential Equations, is a nice one to the audience consisting of graduate students who had taken the standard first-year analysis courses but who had little background in PDE. The later one makes fairly free use of the techniques of real and complex analysis.
Could any expert here suggest a PDE book in the level between these two ones, or a complementary reading for Folland's book?
I don't know if this is a appropriate question here — I have no idea if the question is too localized, I learned from the internet that these two books are very classic for PDE though. Any suggestions for a good update of the question will be really appreciated.
Best Answer
Fritz John's book "Partial Differential Equations" is one of the more elementary but still substantial PDE books I have run across.