See glossaries and definitions in Books on Mac
If you’re reading an interactive book that has a glossary, you can look up the glossary entries.
In the Books app on your Mac, click Library in the toolbar.
Double-click an interactive book that has a glossary.
Choose View > Show Glossary.
If the book doesn’t have a glossary, the command is dimmed.
Click a glossary term.
To quickly see the definition of a word, just double-click it. You can also select the word, then choose Look Up, Search Book, Search Web, or Search Wikipedia. See Look up words.