Early in my career these books were the most helpful to me as a self taught developer:

* CSS Pocket Reference - Eric Meyer - https://www.amazon.com/CSS-Pocket-Reference-Visual-Presentat...

* XML Schema - Priscilla Walmsley - https://www.amazon.com/Definitive-XML-Schema-Priscilla-Walms...

* DOM Scripting - Jeremy Keith - https://www.amazon.com/DOM-Scripting-Design-JavaScript-Docum...