For any adult coloring book fan, the most logical place to start is with getting the right book!
Johanna Basford's books:
They're popular both for their whimsical designs, and for the high-quality materials that the books are printed on. While the books were sold out for months on end, they're now retailing for $9-$11.00 on retailers like Amazon.com.
Secret Garden: Amazon | Indiebound | Book Depository | Barnes and Noble
Lost Ocean: Amazon | Indiebound | Book Depository | Barnes and Noble
Secret Garden Journals: Amazon | Indiebound | Book Depository