Scrum & Agile cheat-sheet, Refcardz
I love to read cheat sheets instead of thick books. I always keep cheat sheets in print or in mobile to read when I am travelling. This helps to utilise travelling time in a constructive way.
Cheatsheets also help me to brush up my already learned concepts that way some concepts always remains on top of my memory stack.
After clearing my scrum training and scrum master exam, All concepts were clear but we need to visit them after every 3 months to keep them fresh and alive :-)