Honestly, CEP development information is rather scattered all over the place. However, few resources stand out:
1. Official Adobe's CEP-Resource Github page GitHub - Adobe-CEP/CEP-Resources: Getting Started with Adobe CC 2014 Extension SDK that contains all necessary info to develop HTML panels. Unfortunately, it's rather hard to follow if you are new to CEP development. Best way is to download their example scripts and take them apart GitHub - Adobe-CEP/Samples: Samples for Adobe Common Extensibility Platform (CEP)
2. All hands down - best down-to-earth info you can find on Davide_Barranca's blog https://www.davidebarranca.com/ - David host few neat tricks to work with CEP. Also his book Photoshop HTML Panels is like a bible - so far I find he's resources being a must-have.
3. This forum / Google search are your best friend in this case.
However, if you are new to scripting, I'd recommend you looking into ScriptUI panel for photoshop - learn building your tool in it, and then expand it to HTML panel. Because HTML panel is nothing more but a nice-lookiing-interface in photoshop. Fo ScriptUI help check out Peter Kahrel's pdf file ScriptUI For Dummies https://indd.adobe.com/view/a0207571-ff5b-4bbf-a540-07079bd21d75
Other then that - google, forum search, pray, google, forum search, repeat.