My favourite Mathematics books are probable

Apostol -- Mathematical Analysis

Hardy and Wright -- The Theory of Numbers

Kelley -- General Toplogy

Dirac -- Introduction to Quantum Mechanics

Minsky -- Finite and Infinite Machines (almost)

at least in the learning area.  For real mathematics very little beats
Charles Delaunay's Theorie du Mouvement de la Lune.  I never managed
tp read beyond chapter 2 or 3, but it is a lovely book.  I still have
Principia Mathematica on my shelves but it is hard going.

None of these seem very useful for music though.......