A sequence is a series of multiple posts on Less Wrong on the same topic, to coherently and fully explore a particular thesis. See the Library page for a list of LessWrong sequences in their modern form.
Rationality: From AI to Zombies is an ebook collecting six books worth of essays on the science and philosophy of human rationality. It's one of the best places to start for people who want to better understand topics that crop up on Less Wrong, such as cognitive bias, the map-territory distinction, meta-ethics, and existential risk.
The ebook can be downloaded on a "pay-what-you-want" basis from intelligence.org. Its six books in turn break down into twenty-six sections:
The following collections of essays come from the original sequences, an earlier version of much of the material from Rationality: From AI to Zombies:
Ethical Injunctions: A discussion of prohibitions you may want to follow even when you've thought of a clever reason to think they don't apply.
The Metaethics Sequence: A longer version of "Value Theory", discussing the apparent "arbitrariness" of human morality.
The Fun Theory Sequence: A discussion of the complexity of human value, and what the universe might look like if everything were much, much better. Fun theory is the optimistic, far-future-oriented part of value theory, asking: How much fun is there in the universe; will we ever run out of fun; are we having fun yet; could we be having more fun?
The Quantum Physics Sequence: A much longer version of the "Quantum Physics and Many Worlds", delving more into the implications of physics for our concepts of personal identity and time.
Other collections from the same time period (2006-2009) include:
Priming and Implicit Association. Priming may be described as the capability of any random stimulus to commandeer your thinking and judgement for the next several minutes. Scared? Don't be. There exist ways to defend yourself against these kinds of intrusions, and there are even methods to harness them into useful testing mechanisms.
The Science of Winning at Life. This sequence summarizes scientifically-backed advice for "winning" at everyday life: in one's productivity, in one's relationships, in one's emotions, etc. Each post concludes with footnotes and a long list of references from the academic literature.
Rationality and Philosophy. This sequence explains how intuitions are used in mainstream philosophy and what the science of intuitions suggests about how intuitions should be used in philosophy.
Decision Theory of Newcomblike Problems. Decisions need to be modeled with some structure in order to be scrutinized and systematically improved; simply "intuiting" the answers to decision problems by ad-hoc methods is not conducive to thorough analysis. For this, we formulate decision theories. This sequence, themed with an analysis of Newcomb's problem, is a consolidated summary and context for the many decision theory discussions found on LessWrong at the time of writing.
Living Luminously. Luminosity, as used here, is self-awareness. A luminous mental state is one that you have and know that you have. It could be an emotion, a belief or alief, a disposition, a quale, a memory - anything that might happen or be stored in your brain. What's going on in your head?
Benito's Guide aims to systematically fill the reader in on the most important ideas discussed on LessWrong (not just in the sequences). It also begins with a series of videos, which are a friendly introduction, and useful if you enjoy talks and interviews.