I wanted to know if anybody has tried dating the book 'Shri Shani Mahatmya'? Is there a version in Hindi/English available online?
The Shani Mahatmya is a Marathi folktale and according to this website, it's "not from Vedas, puranas, upanishads, or any vedic sanskrit text". But apart from that, I'm not sure about its dating.
In any case, as far as I know the only English translation of the Shani Mahatmya is Anand Gopte's translation, which you can read here.