As the title says, looking for a book to read and this subject caught my interest. Preferably a book that includes both ruthlessness and his widespread impact. Thanks in advance for any ideas.
Before I get going can I ask whether you're interested in a biography style book or a more general exploration of the Mongols around the time of Genghis Khan?
Also while the book /u/PopeOnABomb recommended is a very fun read it does have a few issues.
A true scholar of the field can give you more direction, but Jack Weatherford's "Genghis Khan and the Making of the Modern World" is a good place to start. Like most history books, there are surely debates about certain views or accuracy, but it is a good introduction. I offer this simply because "best" seems subjective. So I have offered a good starting point instead. Lots of what you seek is in here.