His paintings have appeared on the covers of more than 350 books and magazines, including many Stephen King novels, most of the Del Rey editions of Anne McCaffrey's Dragonriders of Pern series, Piers Anthony's Incarnations of Immortality series, the Del Rey edition of Edgar Rice Burroughs' Mars series, Melanie Rawn's Dragon Prince and Dragon Star series, the Del Rey editions of H. P. Lovecraft's short story collections, the DAW editions of Michael Moorcock's Elric of Melniboné books, numerous DAW editions of C. J. Cherryh's work, many of Robert A. Heinlein's novels including Friday and The Cat Who Walks Through Walls, the Ace editions of H. Beam Piper's Fuzzy novels, and Tad Williams's Memory, Sorrow, and Thorn, Otherland, and Shadowmarch series and Brandon Sanderson's The Stormlight Archive. Whelan provided covers and interior illustrations for Stephen King's The Gunslinger and The Dark Tower, the first and last of his Dark Tower books.

In the artist’s own words:

I have been interested in the imagery of fantasy since early childhood and all my artwork is, at its most fundamental level, about creating a “sense of wonder”.  In my illustration, my primary concern has been to create a window into the themes and story elements of a particular book.  My non-commissioned work is concerned with more personal themes.

Stylistically my art is best described as “Imaginative Realism”.  Most closely allied to the scope and feeling of what is referred to as contemporary “Visionary Art”, my gallery art is intentionally imbued with a strong sense of the mystical or dreamlike, and is suffused with symbolic content.  There is a deliberate attempt to invest the image with layers of meaning while having an immediate  subjective or emotional impression.