ARIES: A Transaction Recovery Method Supporting Fine-Granularity Locking and Partial Rollbacks Using Write-Ahead Logging. C. Mohan et al., 1992. The canonical WAL paper; introduces the redo–undo discipline still used everywhere.
The Log-Structured Merge-Tree (LSM-Tree). P. O'Neil et al., 1996. Section 2 motivates why a separate sequential log is necessary even for in-memory writes.