BW Trees are pretty neat [1][2]. Typically BTrees in database engines need latches to prevent concurrent writes messing up the tree but BW Trees conveniently sidestep that requirement by appending all node updates to a linked list (called a Delta Chain), and having each reader apply the delta of updates by traversing the linked list until it hits the final node in the list. Periodically, these linked lists are compacted back into single nodes.

If you like these kinds of data structures, the Database Internals [3] book has a good introduction and explanation of a handful of BTree optimizations like this.

1: https://www.microsoft.com/en-us/research/wp-content/uploads/...

2: https://www.cs.cmu.edu/~huanche1/publications/open_bwtree.pd...

3: https://www.databass.dev/