I've been studying the elliptic regularity theory using $H^s$ spaces as done in Folland's "Introduction to Partial Differential Equations".
At the end of section C, chapter 6, Folland affirms that we have similar theorems using $W^{m,p}$ spaces for $p\in (1,+\infty)$, even though their proofs are more involved.
Unfortunately, he does not provide any reference.
Can you point me to a book where these topics are covered?