I read this ~4 years ago, and the title was shortish.
Basically, Obi-Wan gets sent back in time by the Force, and starts changing history. He saves Siri's life, and starts a relationship with her.
He trains really hard, and eventually gets to the point of being able to beat Yoda in a fight. I remember at the start of the fight, he takes his long robe off and puts it on the floor, and Yoda tells him to put it back on because it conceals his footwork.
When he fights General Grievous, he crushes all four Magna guards instead of only three, which Grievous is impressed by, and it causes him to fight differently against Kenobi.
I have a feeling he told Yoda about the Sith Lord's identity.
Also, the story was completed and a sequel was started (but I'm not sure if it was finished), at some point, which involved 2 blue Grievous clones which apparently learned from fighting experience.