In this link, there are books from David Frawley. He is an expert about Sanskrit mantras. There are 14 books from him in here. Some of his books that I think are most important are 
#1  Mantra Yoga and Primal Sound - Secrets of Seed (Bija) Mantras
#2  Ayurvedic Astrology - Self Healing Through the Stars
#3  Vedantic Meditation - Lighting the Flame of Awareness
This 3rd book is not a PDF, it is an Epub file so you will need a special program to read it.  In this PDFs and Ebooks folder, there is a program you can download that will allow you to read the Epub file books on a computer. Or if you are using a phone, you can get the ReadEra app that can read Epub files. The symbol of the ReadEra app is an orange book with a blue background.
      https://mega.nz/folder/TMMzxYIT#2RHnCQWzXdpqVtjNvSow0w    
There are also many other books in there.