What you actually have, if indeed it's a 4.2mp image, is a 1D (no "S") Mark 1. Owned one for about a year, used it with a Sigma 70-200 2.8 EX lens. Outstanding camera.
To your question...the increased sensor size combined with the larger pixel pitch makes that camera perform better than most 6mp cams. Enlargement CAN be an issue, but at much larger sizes than concern your book. I've seen gorgeous 11x14 double truck images (that's 14x22 btw) produced from both the Canon D30 (the old one, before the 10D) and the Fuji S1 Pro. And both of those cams were 3mp native res.
It's an easy, cheap test. Take one of your best shots from the 1D and crop (actually, a select - copy - paste into new image) a 620x930 section from the 2464x1648 original, save it to disk and have it printed at your local photo lab as a 4x6" print. That'll give you the same ppi count (154) as a 9.5x14 from the original, and therefore the same visual quality. Hold at arm's length and evaluate for sharpness and noise grain. Bear in mind that 11x14 will require cropping from the long end of the image in some manner as that camera is 3:2, and produces an 11x16 proportion. The 1D images tolerate a rather large degree of both up-res and sharpening without image degradation, especially if shot in RAW.
For the double truck, if you choose to do that instead of having them rotate the book, I'd suggest an up-res to 4000 on the long side, and then draw a selection to 2002 wide for both left and right sides, and print each as 11x14.
Bottom line, you get the shot, she'll print up just fine.