Pugh, W. (1990). "Skip Lists: A Probabilistic Alternative to Balanced Trees."CACM 33(6). The original paper; six pages, very readable.
https://www.cs.umd.edu/~pugh/galileo/papers/CACM_Skiplist_1990.pdf
LevelDB MemTable source — skip list with a single allocator arena.
https://github.com/google/leveldb/blob/main/db/skiplist.h
RocksDB InlineSkipList — production skip list with per-node tail allocation.
https://github.com/facebook/rocksdb/blob/main/memtable/inlineskiplist.h
Redis t_zset.c — skip list with per-node span field for O(log n) rank queries.
https://github.com/redis/redis/blob/unstable/src/t_zset.c