New users of SQLAlchemy, as well as veterans of older SQLAlchemy release series, should start with the SQLAlchemy Unified Tutorial, which covers everything an Alchemist needs to know when using the ORM or just Core.
Users coming from older versions of SQLAlchemy, especially those transitioning from the 1.x style of working, will want to review this documentation.
Reference and How To
SQLAlchemy ORM - Detailed guides and API reference for using the ORM
Extending the ORM: ORM Events and Internals
Other: Introduction to Examples
SQLAlchemy Core - Detailed guides and API reference for working with Core