Nozick, a consummate philosopher in the analytical tradition, addresses the central issue of philosophy itself. What is the nature of rationality? If Man is the rational animal that philosophers claim, what are the principles, features, properties, methods, functions, and purposes of reason itself? Nozick concedes any attempt to ground "reasoning" fails, and all reasons for reason are circular, but not viciously circular.
For the brevity of the book, Nozick covers considerable territory. He discusses how reason itself functions and the functions themselves (interpersonal, intellectual, overcoming temptation, investment, symbolic utility, and teleological devices), using decision-value (the most technical topic), Newcomb's Problem, Prisoners' Dilemma, and other distinctions, to explicate how one arrives at rational belief, the reasons we want rational beliefs, and some rules to obtain it.
The most interesting (and disappointing) chapter is on evolutionary considerations. Few philosophers to date raise the specter of evolution at all (unless it is the topic), when, as Nozick rightly suggests, it may have its own overriding features and its own reasons and justifications. He's clearly on to an important facet and introduces issues that "limit" the need for rationality as well as require it.
My principal cavil is that he treats natural selection as a purposive agent without any disclaimers or caveats. Worse, his natural selection's purposive agency is, of course, teleological. First, that's bad form, and second, it's bad (actually wrong) evolutionary science. A subsidiary cavil is that evolution becomes a "rug" under which a-rational, even irrational, decisions may be swept (which may be true, if he is not persuasive).
Ultimately, "a rational decision will maximise an action's decision-value, which is a weighted sum of its causal, evidential, and symbolic utility" (137, passim). And, while rationality is predominately instrumental, it is not exclusively instrumental, giving excellent exemptions and reasons for them.
He considers the effect of biases, preferences ("it is a function of the preferences and believes to be rationally coherent and approximately true [and minimally consistent], and it also is a function of the mechanisms that produce such believes and preferences to produce things like that, with those functions" ), reflexivity, interpretation, conditionalization, probability, philosophical heuristics, and imagination on the outcomes, regardless of the cause.
Overall the book succeeds admirably in capturing the nature of rationality, those features and functions which we expect it to have, why they are important, why rationality remains important for everyone (not just philosophers), some basic rules to achieve it, principles to guide us, and its purposes in human life. He does so with economy, clarity, coherence, consistency, always reflexively to determine necessity and sufficiency. His presentation is paragon for doing and writing philosophy well.