Most people agree that Secret Garden played a huge role in re-introducing the majority of the adult world to coloring. It wasn't the first coloring book aimed at adults, but it was definitely one of the most popular. Johanna Basford has published more coloring books since, but this is the one most people know her for.
As with her other books, Secret Garden is themed. All of the illustrations include flowers, nature, and other garden imagery. Some pages depict more realistic scenes, such as an overgrown shed and orchard. Others incorporate elements of nature into mandalas and other designs.
Secret Garden is definitely one of the more expensive books you can buy. However, the quality is definitely better than most dollar store books or the cheaper books on Amazon. Now there are so many different versions of this coloring book that you should be able to find a good balance between quality and price.
The illustrations themselves are top-notch. The intricate details turn every page into a little puzzle, and completing one of the double page scenes feels especially rewarding. It's fun to find the little animals and symbols that are tucked all throughout the drawings.
The paper quality is really great for most versions of this book (avoid the cheap reproductions from discount sites). Most versions user artist-grade paper with extra tooth to pick up a lot of pigment from colored pencils. Some of the most recent publications have bleed-proof paper that are specially designed for pen and marker coloring. Prismacolors are always a great choice for Secret Garden or any other Johanna Basford book.
Even though the book includes double-sided pages, you shouldn't have any trouble with lighter mediums bleeding through or ruining the texture of the back-side illustration. However, If you want to remove and frame two images that are each on opposite sides of the same page, you're going to have to pick one.