First, it's a totally different beast. 'The Bourne Legacy' novel is Eric Van Lustbader taking over after Robert Ludlum, and it kinda picks up the shattered pieces after 'The Bourne Ultimatum'. But forget Matt Damon's amnesiac hero; this Jason Bourne remembers everything, and the story jumps forward in time. He's older, more established, and the plot revolves around this international finance conspiracy with a villain named Khan. If you go in expecting the frantic, grounded pacing of the movies, you'll be thrown. It's more of a globe-trotting, techno-thriller with a different rhythm.
Honestly, my biggest hurdle was the shift in Jason's character. He's less vulnerable, more like an unstoppable force, which for me lost some of the original trilogy's compelling internal conflict. The connection to the movies is basically nonexistent; the 2012 film with Jeremy Renner just borrowed the title and the Treadstone backdrop for a new character. So you're not getting 'what happened next' in the film continuity at all.
Is it worth it? Maybe if you're a completionist for the book series and curious about Lustbader's direction. The prose is serviceable for the genre, and there are some intricate set pieces. But I found myself skimming the denser financial machinations, waiting for the Bourne I remembered from the earlier books and films to show up. He never quite did, at least not for me.