Illuminations about Gayatri mantrams written by great souls such as Srila Vishvanatha Chakravarti, Srila Bhaktivinod Thakur, Srila Sridhar Maharaj, Srila Puri Maharaj, Srila Bhakti Nandan Swami Maharaj and others
You can order the book from here:
https://rasbiharilal.com/products/sri-gayatri-mantrartha-dipika