I put this together for my own use, but maybe others will benefit and suggest other resources.
With the I7 IDE package you get:
Andrew Plotkin updated Sean Barret's index for Writing with Inform:
Two more manuals are:
Aaron Reed wrote a book on I7:
It's helpful to read the source for other games. Here's an IFDB search for well-rated games with source available:
Jason McIntosh wrote an extension search that will find search terms in extension documentation:
Oliver Reiser made an I7 cheatsheet:
Emily Short wrote a document translating I6 concepts and syntax to I7. It's old but a quite useful birds-eye view in my opinion:
If you compile any game in the I7 IDE there's a good syntax reference in the Index/Phrases tab. There's an older plain text reference but you should prefer the reference in the IDE.
Emily Short also made a rules flowchart:
I7 includes the Standard Rules with every game. It's a great resource for learning I7 and understanding a game's default behavior:
You can sort of think of Inform 7 as a domain-specific language written in Inform 6 (which itself is a DSL, but anyway), so it's helpful to understand some I6 eventually.
If you include visuals and sound, you may need a beginner's reference for Glulx: