It is hard to figure out where to start. Nearly any learning-your-self-type of sites will be useful. I used Code Academy and found it fun and useful (they kind of gamify things, making you want to get more "accomplishments" which helps me sometimes!). I like Tim Witt's answer in that thread and used that Build your first application website.
I also found that just modifying existing stuff helped me a lot too, sort of seeing what is already there and what kind of things I can do to make good changes. Playing around in the "Sandbox" is good too.
I have the Getting to Know Web GIS but I've been too busy to read it, unfortunately. I also obtained this book: Web GIS | Principles and Applications and still need to go through it. It's a forever learning process!
I hope this is helpful!
I would like to start learning how to customize apps and web pages.
ArcGIS Online http://www.arcgis.com/home/index.html gives you many options for "configuring" maps/apps/web pages without having a strong JS/HTML background. You can sign up for a free account (start http://www.arcgis.com/features/features.html ) if your organization doesn't have one already.
You also have an option to configure AND customize using the Web AppBuilder developer edition. The user comunity has quite a few custom apps that allow you to extend the out-of-the-box (OOTB) version with little or no progamming skills, and this is a great way to learn how others program. It is a great way to learn (i.e. learning to read code) and a good programmer learns to not reinvent the wheel but massage or improve on it. I have a series of blogs that will help with WAB developer edition, and the latest version WAB 2.5 was just released today (7/19/2017), so good timing on that.
/blogs/myAlaskaGIS/2016/03/09/web-appbuilder-developer-edition-customization-resource-list?sr=search... main list and has links to the other blogs,
/blogs/myAlaskaGIS/2017/03/04/web-appbuilder-the-custom-widgets-list-332017?sr=search&searchId=9944b... to see some of the customization others have done. With the 2.5 release, some of these custom widgets may now be part of the core....too early to know.
Watch the recorded tech session videos, do some of the free online classes, etc. I've been doing this development for years and I would definitely not call myself an expert and any of it...but learning where you can find the answers online, and using Geonet when you get stuck (hint...make sure to post in the correct location, include software and development environment and version,follow https://community.esri.com/people/curtvprice/blog/2014/09/25/posting-code-blocks-in-the-new-geonet?s... when asking questions, etc)