11 Million colorists can't be wrong! Secret Garden is the original adult coloring book by Johanna Basford. It can be credited with helping to bring the wonderfully relaxing and creative activity of coloring back to the lives of grownups.
Her drawings have a distinctive style - lots of small, detailed pen-and-ink illustrations filling the pages. Part of the fun is the bevy of little garden creatures hidden among the flowers and scenery. There's a key and index in the back of the book if you want to make sure you found all these little playmates. Her books are also known for making every surface colorable - even the covers and book jacket (inside and out!)
There is quite a variety in the drawings - from multi-page spreads to round images reminiscent of mandalas, but filled with garden forms rather than geometric patters.
Recently the publisher has upgraded the paper stock to one that is thicker and heavier, helping to prevent bleed through from pens and markers. This has been a major complaint from many customers, as the pages are printed on both sides. Although the publisher recommended felt tip pens, many found their colors bleeding through and ruining the back side illustrations. Pencils of course don't have that problem, but there are a lot of fine details, and a lot of colorists find those tiny areas are easier to color with a fine-tipped pen or marker.