Mandalas are widely known as symbols that assist with meditation and concentration, but there are complexities and profundities of meaning, if you want to dig a little deeper.
Mandala, by Judith Cornell
This insightful book has a wealth of useful and interesting mandala-related information, along with many exercises for creating your own different kinds of mandalas.
Mystery of Mandalas, by Heita Copony
This short book was published by The Theosophical Society.
Meditating with Mandalas: 52 New Mandalas to Help You Grow in Peace and Awareness, by David Fontana
There’s a ton of introductory information about mandalas, followed by 52 original colored mandalas with suggestions for exploring each one as a help to meditation, contemplation, concentration.
Natural Mandalas: 30 New Meditations to Help You Find Peace and Awareness in the Beauty of Nature, by Lisa Tenzin-Dolma
This book is in a similar format to the one above, except these mandalas are all nature-related.
Zen Doodling Mandalas, by Carolyn Franklin
This is a beautiful and fun book that shows you how to make your own mandalas, including some truly unconventional ones.