The Go to Page command only works within a document. The Go to Page in Book command would open up the requested page in the Book file, regardless of whether the document were open at the time or not.

This is a valuable command because when editing long books with many chapters, it is useful to go directly to a page to make a change. It saves having to take your hand off the keyboard to the mouse, open the possibly closed Book panel, find the file that is in the page range, double-click it to open it, and then execute Go to Page.