Baltasar Gracián was a 17th Century Spanish priest who wrote proto-existentialist philosophical essays and prose. This collection contains a selection of aphorisms that are akin to the Italian Machiavellian
The Prince, offering advice on how to exploit your friends, your enemies, your talents and other people's flaws.
The advice in this little black book is something that hasn't changed at all since it was written, and all of it is still as relevant today as it was then. It is full of contradiction, however, because I firmly believe in the mantra "be yourself", but the advice tells you to change your personality to fit every single occasion in order to gain advantage over everyone in every situation. The advice that does not ask you to do this is well worth adhering to, however.
Like all of these Little Black Books, it is written so well and thoughtfully that I cannot fathom how anyone could not fall in love immediately.