The best index of RPG.net Let’s Read discussions can be found on the web site Taxidermic Owlbear . There’s some great ones, like the Monstrous Compendium discussions, very much worth reading, especially if you’re curious about some of the older D&D weirdness.
As I write this post, there’s about a hundred “Let’s Read” submissions cataloged for a range of game systems. D&D editions include Basic, Advanced 1st edition, Advanced 2nd edition, Advanced 1st edition, D&D 3rd edition, Pathfinder, D&D 4th edition, D&D 5th edition, Legend of the Five Rings, and more.
The ‘format’ for these is a discussion. Whomever starts the a thread for the discussion and reads a section every so often, summarizing and discussing it. Others may comment if they want, adding their own opinions and observations as well as asking for questions. It’s a mix of a review, a detailed analysis of mechanics and game design, and a celebration of older game materials.