Hi Joost & Visvanath,
Thanks for sharing!
If I were to recommend one book only for instructional design, it'd have to be:
Not for online learning per se but easily applicable. Both authors are based at the Open University of the Netherlands so very familiar with technology enhanced learning & elearning.
Also, this one's a quick, easy to read introduction to the learning sciences. You can read it in a couple of hours & it'll give you an overview of what you need to know & where to look:
I hope this helps!