As with most things, the Wikipedia entry is a good starting point which is likely to keep up-to-date: “Alt-right”

“Ctrl-Alt-Delete: The origins and ideology of the Alternative Right” by Matthew N. Lyons is a thorough and critical overview of the alt-right, much of which is provided in various alt-right personalities' own words. See also his “Calling them ‘alt-right’ helps us fight them” which attempts to differentiate the alt-right from previous white nationalist movements.

M. Ambedkar’s “The Aesthetics of the Alt-Right” is an in-depth look at the alt-right provided through a two-axis typology (economic nationalism, white nationalism, esotericism, accelerationism) and a focus on their online artwork.

Vox's guide to the alt-right focuses on its progenitors (paleoconservatism and neoreaction): “The alt-right is more than warmed-over white supremacy. It’s that, but way way weirder.”

For a more friendly view, read “An Establishment Conservative’s Guide To The Alt-Right” written by Allum Bokhari and Milo Yiannopoulos for Breitbart. (Update: BuzzFeed published a report based on some of Yiannopoulos’s leaked emails which gives insight into how this article was built, and generally how Yiannopoulos/Breitbart acted as a sort of interface between the openly white nationalist alt-right and the more traditionally conservative ‘alt-lite’: Here’s How Breitbart And Milo Smuggled Nazi and White Nationalist Ideas Into The Mainstream.)

The best resource for understanding the alt-right in the white nationalist/race-realist’s own words is The Alt-Right FAQ.

Andrew Marantz’s New Yorker article, “Trolls for Trump,” is not an alt-right guide. It is a narrated interview with a single Twitter user, but it could serve as the closest thing to an ethnography of the alt-right that anyone would ever want to read.