ChessFromScratch

You can use a chess library and never learn what it is doing. Castling rights, en passant, move legality under check — they are all handled for you, and all of the interesting difficulty is hidden.

Chess From Scratch is a complete chess engine built bottom-up — rules, board, moves, and an engine to think with — written without chess libraries to understand how engines really work.

So this one is written bottom-up with no chess libraries at all: board representation, the full legal-move rules, move generation, and a search that picks a move. The point was to understand engines by having to build one.