About 1/2 of the book covers Althusser's relation to the French communist party, Stalin, Mao, and competing philosophers. The other 1/2 covers his philosophy, and it covers it well. Althusser's writing is, in my opinion, unnecessarily polemical, verbose, and rarely as ingenious as his presentation would suggest. Elliott is able to summarize, without losing much neither philosophical force nor consistency, For Marx and Reading Capital, along with placing them in their historical context. Despite being an anti-anti-Althusserian, Elliot reached the same conclusions I did (having read only For Marx, not RC): Althusser's project was mostly a failure, and each time he tried to get off the ground, he leaped into a glaring contradiction (e.g., Marxism is a science, why's that? Because it is!). The only odd part is the final chapter where Elliott spends ample time suggesting that Althusser was more successful than the previous 4 chapters suggest. Basically his defense of Althusser felt a little forced…
Elliott also spends ample detail explaining why Althusser was so averse to humanist readings of Marx; the ones I'm partial to. Also, why Althusser wanted to purge Hegel from all things Marxism. Again, in the end, Althusser's overall project was a failure, but like Rawls, one can't write a paper on Marxism (or in Rawls's case Liberalism), without having to cite the territory erroneously covered by either thinker.
The only redeeming work from Althusser was his essay on Ideology, which still is as forceful today as it was then. Nonetheless, Althusser bent-the-stick too far away from Humanism, and in the end, gave Marxist humanist even more confidence in their antithetical project...
If you want to know who Althusser was, and what he thought, without undergoing the laborious research project that Elliott clearly undertook, read Elliott.