Best resolution for a picture book?

My father is planning to publish a book about sea shells. He assigned me for the picture taking. What is the best resolution I should set on my camera considering I got 24 megapixels with my Nikon D3200. Also, the pictures will be re-sized and lay-out in a pattern of 4 pictures per page with shells' description at the bottom of the pictures. I'm thinking that if I set it at 24mp and if re-sized the fine details of the shells will be lost.

I'm not a professional, the book he needs to publish is just for his research. Kindly help out.

Shoot with the camera at the highest resolution possible and at the lowest ISO that will remove any camera shake, and under the best lighting possible (perhaps use a light tent).

For printing, the photos are best set using a photo editor at 300dpi and at the desired final width with medium JPG compression. You may need to make some enhancements in Photoshop in terms of sharpness (USM often helps for printing), lightness and saturation. A book publisher's art dept will do all this for you from the original but if self-publishing it more likely has to be done yourself.

If they require images converted from RGB to CMYK format then you'll need access to Photoshop CS (subscription based now), or buy an old licensed copy of the main Adobe Photoshop program. You can convert to CMYK using the free GIMP editor but when I last tried it it was a pain.

120x50

You don't say what size each photo is, but if they are 4x6 and you print them at 300 dpi, that makes them 1200 x 1800, or 2.16 MP. So you see, you don't have a problem.

ALWAYS shoot at the highest resolution.

There are plenty of ways to make a big picture smaller and to reduce detail, the reverse is virtually impossible.