Jan, I have two Gloria Loughman books. Quilted Symphony has five pages dedicated to fabric painting, not specifically skies, but I can see how her painting applies to skies. In particular, she discusses fabric paint, brushes, fabric, painting boards. She also goes into one page of steps on how to apply the paint; she also discusses color gradations and creating textured fabrics (these last two sections are very sketchy).
I also have Luminous Landscapes. In this book, she dedicates seven pages to fabric painting and MOST of it is dedicated to skies. If I were to buy one book with skies in mind, it would be this book. The book goes into many more aspects of landscape design, including skies than just the painting aspect. You might be able to go online to Amazon and look at a sample of the book.
Hope this helps!