Reference Request: Mean Value Property over a ball in $R^n$ in Standard Coordinates

Question

I am looking for a good reference on dealing with the Laplace equation rigorously. In particular I am looking for:

• Proof of The Mean Value Property in $R^n$ using Green's Second Identity.
• Proof of The Maximum Property
• Derivation of the Poisson Formula

Any book/notes/pdfs/webpaces/etc. would be greatly appreciated.