Probably my favorite Batman villains is Ra's al Ghul (roughly translating to Head of the Demon), and second to only Deathstroke in DC as a whole, the Demon Trilogy is one of my favorite reads. This immortal ecoterrorist has been a calculating, but elusive villain since the 1970'sand in the Demon Trilogy, we get three iconic Ra's al Ghul stories: Bride of the Demon, Son of the Demon, and Birth of the Demon.
Bride of the Demon has Ra's al Ghul finding a new love and hoping to end Batman once and for all. This is also the first time where Ra's al Ghul and Tim Drake (then a Robin in training) meet. The two would have a few encounters over the years, my favorite being in Christopher Yost and Ramon Bachs' Red Robin series (along with Fabien Nicieza's as a writer and Markus To as the major artist after the first six issues).
Son of the Demon has Batman and Ra's al Ghul teaming up to stop a terrorist from unleashing a powerful weapon. This title itself makes a bit of sense when considering how Ra's sort of sees Bruce/Batman as a sort of son (or at the very least a worthy heir) and is one of the rare times that Batman and Ra's work together. This story also dabbles into what would later result in the origin/birth of Talia and Bruce's son, and current Robin, Damian. Son of the Demon also explores Bruce and Talia's (Ra's' daughter) romance once more.
In Birth of the Demon, we get what most would consider Ra's al Ghul's definitive origin. Here we are introduced to a younger Ra's al Ghul roughly six to seven hundred years ago. Before he was even called Ra' al Ghul. Here he was a member of a nomadic tribe and it discusses how he came across the Lazarus Pit, his first love Sora and her demise, the King who came to him for aid, and Ra's descent into becoming the villain he is today.
If you enjoy Batman and/or his rogues gallery, the Demon Trilogy is a set of stories I would recommend. Especially if you enjoy this particular character. It can be found in the adult graphic novel section.