The artists I’ve worked with have a policy that you will own the artwork but the artist retains the copyright. That means that the artist will provide you with the artwork to use for your book and all book-related materials (press kit, marketing materials, bookmarks, author website, etc.). You will also have exclusive use of the artwork for your book, and the artist will not sell it to anyone else or use it for any other purpose.
The artist will, however, retain copyright of the artwork. This means that the artist has the right to show the artwork to prospective clients as an example of their work, and include the artwork in their portfolio.