In the "For Further Reading" section of my history textbook there are two books related to Ashoka (while I haven't read either, you can look at them yourself and decide how helpful they are).

-The Legend of King Ashoka: A Study and Translation of the Ashokavadana by John S. Strong

-Ashoka and the Decline of the Mauryas by Romila Thapar

I hope you find either helpful.

EDIT: Formatting