The book that I have used extensively this year is by David Glover called Quarries of Lakeland. Very well written, informative, detailed account of slate workings throughout the Park. Pictures aren't great and some surprising omissions (like Peat Field at Hodge Close), but still an excellent work that I highly recommend.
If you check the Facebook Group called Mines and Quarries of the Lakedistrict, you can see loads of details of people exploring mines and quarries in the area, including Wrengill. I post there as Mark Hatton.