Xah Lee has a blog post (2014-01-16) that reminded me of two useful functions that I can never seem to remember: emacs-index-search and elisp-index-search. They allow you to look up a topic in the Emacs or Elisp manuals by querying the index of those manuals. They aren’t bound to any keys by default (which is probably why I always forget about them) but they are available from the Help menu if your Emacs configuration displays it.
Lee uses elisp-index-search enough that he’s mapped it to a function key. That’s probably overkill for me but with the extraordinarily useful combination of ido-vertical-mode and flx-ido I can just type 【Meta+x】 eis to get the elisp index and 【Meta+x】 emis to get the Emacs index. That’s easy enough that I don’t mind doing it; now all I have to do is remember to use them.