some ideas:
bookmark - same as sidebar icon in the library though
bookmark.square - the bookmark icon is smaller, and square feels just like a border
books.vertical - symbolically makes some sense, as they're books in a line, kinda like bookmarks in bookmarks bar???
tray - used for inbox though
archivebox - too far removed from bookmarks bar maybe
list.bullet - i suppose it works, but not too representative of the actual bar
