It will be nice to have list of authoritative practical mantra shashtra n practice article links online which will help all.
I have compiled a short list of articles and a books related to your enquiry about the reference material about Mantra Shastra Online. Some material is from well known organisation called All World Gayatri Pariwar and well known author Pandit Shriram Sharma Acharya.
- Here is the PDF BOOk - THE ETERNITY OF SOUND AND SCIENCE OF MANTRA – BY PANDIT SHRIRAM SHARMA ACHARYA.
- Here is a PDF Mantra Yoga by Himalayanacademy.com - The topic of science of mantra is discussed in this book.