You need to know Javascript's quirks. I generally recommend [1] for experienced developers. Everything on that page is important to know (edit: except the perf stuff, which changes as the js engines get better). I'll second the Eloquent Javascript recommendation in another comment if you're looking for a more involved book.
With the JS quirks out of the way, I would recommend building something in React and muddle through things with SO/google.
The hardest part of frontend dev for an experienced backend dev is the CSS. CSS is conceptually simple but there's a LOT of random domain knowledge you need to get layouts you want. You can bypass a lot of it by using a sass framework (e.g. foundation or bourbon) at the cost of adding a build step and another abstraction layer on top of CSS.
[1] http://bonsaiden.github.com/JavaScript-Garden/
With the JS quirks out of the way, I would recommend building something in React and muddle through things with SO/google.
The hardest part of frontend dev for an experienced backend dev is the CSS. CSS is conceptually simple but there's a LOT of random domain knowledge you need to get layouts you want. You can bypass a lot of it by using a sass framework (e.g. foundation or bourbon) at the cost of adding a build step and another abstraction layer on top of CSS.