Yes, as Tash said, Grove is the best. However, you will need a bit of shelf space and a large wallet! (29 volumes!

He he

)
More manageable are “The Oxford companion to music” (Ed. by Scholes – Oxford University Press) and the Larousse Encyclopaedia of Music (Ed. by Geoffrey Hindley – Hamlyn). They are only one volume each and cover pretty much everything. I do not know of any similar work dedicated exclusively to the piano, although there is “The Cambridge companion to the piano” (Cambridge University Press) which is very interesting, but not really an encyclopaedia.
Best wishes,
Bernhard.