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.