Not Until You’ve Done Your Practice!  A short, funny and practical guide for students (all levels) and parents with practical ideas for making practice efficient and effective.   By Philip Johnston.  

Practice-o-pedia  is an excellent guide for late intermediate/advanced students who wish to make their practice time more effective.  Also by Philip Johnston). 

Learn Faster, Perform Better covers the neuroscience of practice.  It’s written by a musician with graduate level training in both music and neuroscience; the writing and ideas are clear and compelling.

Learning Strategies for Musical Success is an excellent guide for parents and students (written in layperson’s language).  It applies to pre-college students as well as those pursuing advanced studies.

Interleaved vs. Blocked Practice discusses the pros and cons of mixing sections in practice (interleaved, i.e., mixing up sections, which provides better long term results) and practicing blocked groups sequentially (which provides short term gains without long term reliability).

Musictheory.net offers a number of ear training and reading exercises for beginning through advanced levels (there are two choices on login:  Lessons and Tenuto.  Lessons is the free version,  Tenuto is the paid version.  The Royal Conservatory also has an online theory training program that corresponds with each student level).

The Bulletproof Musician includes a blog, and a complete program of practice for advanced musicians that includes information on managing thoughts as part of performance preparation.