Wendy Piersall is a lifelong writer and artist with over 17 years of professional design experience. She began drawing mandalas for coloring for her Woo! Jr. website for kids and parents, and when Ulysses Press asked her to do a coloring book for adults, she did Coloring Animal Mandalas.
The book did so well that Ulysses asked her for more. As of October of 2015, she has published three more coloring books for adults, Coloring Flower Mandalas, Coloring Dream Mandalas, and the most recent, Coloring Ocean Mandalas.
Each of the books have the drawings on one side of the page. Markers will bleed through, so I recommend putting a couple of blank sheets of paper under the drawing. The mandalas are centered on the pages, and encompass a wide variety of themes, and subjects. The designs vary from simple to complex and will provide many hours of enjoyment.
In addition, Wendy has a YouTube channel that lets you see how she colors some of the mandalas. The videos use time lapse so you can see all of the shading. On her blog, she has an excellent article on how to color (for grown-ups).