I have a few thoughts. I'm not aiming to offend or hurt feelings; I'm an editor, and it's my job to be critical...
1.) The Red Harlot drawing looks far better than the rest of the "book pictures" on the page, and in fact has sharper lines and better shading than anything else on the page. If all the "book pictures" looked sharp, and everything else looked fuzzy, or if all the images (books, tables, foreground, background) were all equally sharp and shaded, it would be fine. But having that one item stand out doesn't work well. I'm sure Photoshop has a "fuzzy" filter that the image could be run through. Some level of consistency (within the entire image, or between the monster art crawling off the pages) is necessary.
2.) The foreground overall works well, but the background needs better definition. The few abstract shapes and lines in the background don't work to ground the images. They're somewhat distracting. This is especially true of the series of sloping boxes that go from upper left to lower right, just behind the title. Something simple, like a filled bookcase in the right-side background, would probably balance the picture well.
3.) Overall, the "creatures crawling off the page" motif works well, and is a nice halfway point between the 3E trend of "books that look like older kinds of books" and actual cover art. I like the theme, and I like the overall flow of the image.
4.) The approach of "title, subtitle" is iffy. I don't mean for this cover only, but in general. Too many titles tend to both crowd the page and impede name recognition. (one memorable example was Mongoose Press' "Encyclopedia Arcane: Constructs - It's Alive!") The "Here there be monsters" logo looks good overall (though I'm not fanatical about the 'o' in monsters...) and the "Veiled Denizens" font and size are balanced on their own, but taken together, it's quite a mouthful: "Here there be Monsters: Veiled Denizens" Gamers love abbreviations, and would probably have no problem with 'HtbM', but I think it's pressing your luck to go out to "HtbM:VD". (and I'm old enough to giggle at anything with the initials 'VD' just on general principle) I know the title issue is probably already a done-deal, but if you have to go with the long title + subtitle, you might consider adding 1/4" to the bottom of the artwork, and moving the subtitle to the bottom of the page and shfiting the title & top border down to where the subtitle was, with the bottom of the title just above the top line of the bookcase.
Taken as a whole, I think it's good art, could be a workable cover as-is, but I think with a few minor changes (harlot art, background definition) it would look considerably more appealing.