JavaScript: The Definitive Guide – this was the book that every front end coder had to own. This book explains javascript and it’s APIs in all their glory and helps you navigate the murky waters of cross browser compatibility.
JavaScript Objects – a book about working with objects in Javascript code from before anyone knew Javascript was the beautiful little language that it is.
Dynamic HTML: The Definitive Reference – dynamic HTML might sound a little bit quaint these days, but it was people pushing the boundaries of what you could do with Javascript and CSS that lead to the magic that happens in browsers today.