Only certain icons actually render in the selection. When using the "book" icon it will just create an empty space where the icon should be.
Topic on Talk:OOUI/Widgets/Icons, Indicators, and Labels
Hi @Cyberpower678, where are you looking at? In order to load certain icons, you need to also load the corresponding icon pack as mentioned for example in https://www.mediawiki.org/wiki/OOUI/Using_OOUI_in_MediaWiki#PHP
That looks like backend documentation. I'm trying to create a JS gadget and I want to load the book icon, but it doesn't render.
You can add $out->addModuleStyles( [ 'oojs-ui.styles.icons-editing-core' ] ); in your moduleloader-hook to load additional icons.
@Cyberpower678 You need to load related styles via ResourceLoader, e.g. mw.loader.load( [ 'oojs-ui.styles.icons-editing-citation' ] ); for book icon.