The Agony and the Ectasy is one of those books that is difficult to put down...reminds me of the narrative writing style of Aynn Rand in the Fountainhead or Atlas Shrugged. This is one of the few historic novels that I would read again.
However, if you are looking for a 'good book' to read, this is not the one. The prose gets repetitive and tired very quickly, the characters sometimes very one-dimensional.
This aside, the book is a great history, if not a great work of art.