An absolute must for players of all levels is "The Poker Mindset" by Ian Taylor and Matthew Hilger. Whilst understanding hand rankings, pot odds, probabilities etc;etc; is important to becoming a winning player, understanding how to deal with bad beats, downswings and tilt is paramount and this book will give you the answers.
I could go on & on about how good this book really is, but i won"t-just buy it! And please do not turn into one of those players that say"s "i"ve got that one at home", but in reality has never read it? Buy it, read it and keep reading it-believe me you will not regret it.
Kevin