Learn German: Die neue Anleitung liest sich wirklich gut.

This is a reflexive construction used as a passive alternative. Instead of using the normal passive to say the manual is read by someone, German uses liest sich to focus on the manual's qualities. English does the exact same thing here: The manual reads well.

How is this different in meaning from the standard passive, like Die Anleitung wird gelesen?

A standard passive like wird gelesen just describes an action happening to the manual at a specific time. The reflexive alternative liest sich describes a permanent characteristic of the manual itself. It implies that the experience of reading it is easy, pleasant, or successful.

Could I just say Die Anleitung liest sich and drop the wirklich gut?

Generally, no. When using the reflexive as a passive alternative, you almost always need an adverb or adjective that tells us how the action unfolds. Words like gut, schlecht, leicht (easily), or schwer (with difficulty) are necessary to complete the meaning.

If this is a passive alternative, could I also use the sich lassen construction here?

Yes! Sich lassen with an infinitive is another great passive alternative. You could rewrite this as Die neue Anleitung lässt sich wirklich gut lesen. Both versions express that the manual can be read well.