Which is the best FREE Scrum Book?
There are many paid resources out there but I do not feel it is always necessary to pay. The scrum guide is good but short. This one I can recommend: http://www.freescrumebook.com
What are your thoughts?
Best scrum books are not free. However. you can start with free The Scrum Guide by ken anf jeff.
The next free resource to learn executive version of scrum is the book A Lightning Introduction to Scrum https://www.smashwords.com/books/view/644497 .