What makes the 20 most spoken languages tick?
I greatly enjoyed the book Babel: Around the World in 20 Languages.
Gaston Dorren explains the histories of the 20 most spoken languages in the world, with unique and funny stories about each one. Read this book if you are not just keen on learning foreign languages, but are also curious about the connections between different languages and how languages evolved.
He gave funny and insightful anecdotes about certain words and how language is used in various countries and contexts. The section on Arabic shows how many words in English actually come from Arabic roots was mostly all new information for me, despite my two attempts to study Arabic. (I even reviewed Rosetta Stone for Arabic and composed a list of free Arabic learning resources.)
I liked the lighthearted way he wrote about the intricacies of various languages. The chapters are short, making it easy to read when you have just short pockets of time.