Musicians, if you are searching for musical scores to help them practice your classical music performance, you can find royalty-free, public-domain sheet music at 
Sheet Music International to help you improve your note reading skills or practice your actual performances
You can also get a little more insight and wisdom into the world of practice and performance with titles like the ones recommended below:
The perfect wrong note: learning to trust your musical self by 
Pumping nylon : the classical guitarist's technique handbook by 
The string instrument owner's handbook by 
PianoLab : an introduction to class piano by 
Harmony at the piano: using keyboard harmony to learn advanced piano music by 
Beethoven sheet music for piano rom easy to intermediate: over 40 masterpieces. by 
Modern etudes and studies for the total percussionist by 
Drum sound and drum tuning : bridging science and creativity by 
How to write for percussion: a comprehensive guide to percussion composition by 
Modern etudes and studies for the total percussionist by 
A dictionary for the modern percussionist and drummer by 
The naked voice: a wholistic approach to singing by 
The owner's manual to the voice: a guide for singers and other professional voice users by 
The essentials of beautiful singing: a three-step kinesthetic approach by 
The heart of vocal harmony: emotional expression in group singing by 
Voice secrets: 100 performance strategies for the advanced singer by 
