This won't be specific to top-down shooter, but totally canvas for sure.
- http://codeincomplete.com/ - multiple resources for multiple games, with in-depth explanation plus, dives into general game development (a little). Have never built something based on it, but from what I've read, that's a decent resource.
- http://nicolahibbert.com/html5-canvas-breakout-game/ - a game developed based on other tutorial ( http://billmill.org/static/canvastutorial/ ) and expanding upon it. Then, a little recap with what this particular developer had troubles with or found troubles with.
These two should be good to start with.
Anyways, Canvas is simply a "graphics" renderer, the game logic behind can be anything gamedev related, you just gotta write in JS and display through Canvas API.
Whether these two are "best resources"... Depends, because, there is no such thing as "the best".