I hosted a small session on the Bitcoin Lightning Network at Hacker Beach last night, and promised to share all the resources from it somewhere. Hence, here's a random list of things you can read to learn about Lightning Network, and also start using/testing it.
Understanding the protocol
* The original Lightning Network paper
* Slides from a Lightning Network talk at the SF Bitcoin Devs meetup (Feb 2015) -- Slightly outdated, but contains a lot of the basics (from the authors of the paper)
* The Lightning RFCs / BOLTs -- This repo contains the various protocol specifications. Also, scroll down in the README for a nice short introduction section.
* Lightning network in depth, part 1: Payment channels
* Lightning network in depth, part 2: HTLC and payment routing
* lightning-onion -- Repo containing the code for LND's onion routing. Also, check the README for a broad overview of what this means for the Lightning Network.
* LND Developer Site -- Resources and documentation for the Lightning Network Daemon (LND)
* c-lightning -- A Lightning Network implementation in C
* Spark Wallet -- A GUI client for c-lighnting
* eclair -- A scala implementation of the Lightning Network
* eclair-mobile -- An Android wallet for the Lightning Network
* Lightning Wallet for Android -- A fork of eclair with support for receiving payments (includes watchtowers)