Everything Old Is New Again

The Guardian and The Millions both recently wrote about the books we love to reread (click here for the piece on The Millions). I have several series of books that will pull me back in every few years or so. It's as if I'm seeing old friends after a long absence.
I don't think I could go so far as to read the same book at the same time every year, but some books just deserve that revisiting.
I started to reread Colleen McCullough's Rome series in anticipation of my trip in May. She has spun magic out of a dry history, bringing to life some of the greatest names in ancient Rome - Pompey, Cicero, Sulla, Marius, Spartacus, and of course, Caesar.
She also does an amazing job of mapping the streets, describing a soldier's life, and detailing the making of laws and governments in a way that keeps you engrossed and turning pages. That's quite a task.
If you haven't read the series, start with The First Man in Rome. I just finished "hanging out" with my favorite bad dictator Sulla in Fortune's Favorites and I'm about to watch the triumvirate between Pompey, Caesar, and Crassus disintegrate into civil war in The October Horse.
What are your favorite old friends in the book world?