A Wiki Sub-Page is an open, public page that will be used to add content that doesn't fall into a more structured content type like Achievements, Reviews, Images, Trailers, etc.
On every Game Page on the site, scroll to the bottom and you will now see a very powerful link called "Add Wiki Sub-Page". Click the link, and you will see the same basic form that you've been using to add Game pages.
You can add whatever you want.
Let that sink in.
After you've changed your pants, you can see the "book" navigation in action on the Halo Reach page. On the left, under the game's name, is a Wiki Sub-Page called Ranks. No more land grabs for tab space like on NGW.
You may want to read about how Wiki Sub-Pages structure themselves.
This is another huge step forward for WikiGameGuides. I hope everybody that has been asking for years to add a guide to NGW or suffered through the Current Projects forum understands the magnitude of this update.
If something doesn't make sense, PLEASE let me know. I'm working on this site (what feels like) 24/7, and anything I can do to make it better, I'm all ears.