Music and text

Yes you can.

Just add a vertical or horizontal frame to your score and then add a text box to that.

Here's the manual page on entering frames: http://musescore.org/en/handbook/frame

A vertical frame will need a horizontal frame adding to it before you can enter text.

You can then type away anything you want to.

You can add graphics too by simply dragging and dropping from the directory where the graphic file is stored.

HTH
Michael

If it's a book that is mostly text with musical examples here and there, you'll probably find it preferable to write the book using a word processor, but create the examples in MuseScore. Then export the examples as PNG files and import them into your processor. There is a nice plugin you can install from the plugin repository (see menu at right pn this page) to simplify the proess of exporting examples as graphics, automatically cropping out all the unused space on the page. It's called Snippet Creator.