Textboxes are a great way to add visual appeal, break up lengthy blocks of text to help readers maintain focus, and highlight certain kinds of information or specific pedagogical features.
Textboxes are best suited to recurring content features that appear in every chapter, but the enclosed content could be considered tangential or supplemental to the points covered in the main body of the writing.
Placing such content in a textbox and using a consistent heading will let the readers know what the box contains and why this has been included.