Layered application files are preferred and should be designed at trim size plus an additional 3/4" off all edges (to allow for bleeds on hardcovers, wraps on spines, etc.). For our standard 6-1/8" x 9" book, that equates to 7-5/8" x 10-1/2" (or 2288 x 3150 pixels). Art smaller than that, or designed without a bleed area, may be presented as an inset.
The fillable form linked in the sidebar asks many of the essential questions needed to review the viability of supplied art.