Coloring books come pre-inked so my children (or whoever is coloring) will never be challenged to use a pen or brush with precision. In addition, I'm a firm believer that a strong background in tracing leads to great drawing abilities.
Not surprisingly, the solution is a coloring book that doesn't come pre-inked. In fact, it could be a whole line of coloring books ranging from typical children-style versions with cartoons to grown-up versions with more complex illustrations. I would be interested in this grown-up version if it had pencils from my favorite artists (or a variety) for sure and perforated pages so you can detach the art and hang it up after you've created it. Perhaps for the uber nerd a variety of styles would also be included to challenge your inking abilities.
Of necessity, the paper quality would need to be suitable for holding ink, and the entire product line would of course not be suitable for very young children.