Books & websites
Leaving Mundania is a book by Lizzie Stark for general audiences covering the phenomenon of larp in North America and beyond. The companion blog is filled with great articles about larping.
Nordiclarp.org is an online magazine covering all things Nordic larp. There are many amazing essays and videos of thought-provoking presentations.
The Nordic Larp Wiki is a great resource about Nordic-style larp games, tools, techniques, and concepts.
Manifestos, movements & schools of thought
At various times people and groups have published (slightly tongue-in-cheek) manifestos declaring a set of creative ideals for a movement. Some of these have been influential and are worth knowing for historic reasons.