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.