After Codecademy got me off the ground with jQuery, I moved on to the jQuery Learning Center put together by the people behind the jQuery project. The learning center assumes some level of existing knowledge, so I was glad I got the Codecademy primer first.
As expected, the jQuery Learning Center went into more depth on topics that were outside of a quick primer such as Codecademy.
Implementation: The learning center gave some details on how jQuery works behind the scenes. This helps programmers understand and debug when things go wrong, and sometimes leads to information on code performance.
Performance: Hand in hand with implementation details, the learning center teaches some basics on writing efficient jQuery code. This is different from writing terse jQuery code. What looks like small amounts of jQuery code might end up triggering a lot of work for the computer behind the scenes. A programmer unaware of the implementation and performance consequences can get tripped up.
History: The learning center occasionally switches into history lesson mode. It is moderately enlightening to understand the evolution to the current implementation. More importantly, it helps people recognize and understand old legacy jQuery code they might end up having to maintain.