Good are transalntion original eastern texts of yoga.
Sat chakra nirupana and The serepent power by A. Avalon (John Woodroffe), Shiva samhita and Gheranda samhita. The Third eye is chakra Ajna in yoga.
And Chinese Taoists text The secret of the golden flower, translantion by Richard Willhelm. But there is the Third eye called "heavenly" Heart.
Is recomended know the Chakrayoga theory before you start.
Those texs are free on net. Mostly in PDF.