This little video from Abebooks is the closest I've ever gotten to flipping through a copy of the Codex Seraphinianus. What a truly weird book.
I particularly love it when the staid narrator reveals his "favorite" illustration - a roller skater murdered by a monstrous pen. What?!
The Codex reminds me of If You're Afraid of the Dark, Remember the Night Rainbow by Cooper Edens. My mom had a copy and I used to flip through it as a child, confused and not a little disturbed. I still took things too literally to appreciate the visual non sequiturs, combined with the nonsensical text ("If you have butterflies in your stomach...ask them into your heart").
The Codex can seem almost plausible as a factual document - it might be an artifact of an alien civilization, or a faithful narrative of an alternate reality with a totally different physics - but then the murdered roller skater yanks you back to a world you know, and you realize the artist, a Roman architect, is our contemporary. (He's actually still alive.) That the book is so disconnected from the recognizable and quotidian reality we all live in is thus both wondrous and creepy.
I think that creepiness is the difference between the lush dragonscapes on the cover of fantasy novels and the paintings of Salvador Dali, or between fairy tales and magical realism. (No wonder that Italo Calvino wrote a foreward to at least one version of the book). There's a lurch, a vertigo, a dissonance created by fantasy that's almost real: the Codex Seraphinianus, like the Voynich Manuscript and some of the truly great works of literary science fiction, occupies a textual Uncanny Valley. I think that's why people covet it so much.
Find copies of Codex Seraphinianus at Abebooks - a really good online network of used bookstores.