Any documentation available that would help developing a Bitcoin Client? I want to start developing a Bitcoin Client in a new language. For this I'm looking for references and resources that could be used to as a guide. Some kind of comprehensive guideline of what a client should include. But haven't been really successful so far. Does anybody knows about something like this?

EDIT:
I'm aiming for a full node with wallet capabilities
The bitcoin.it wiki contains a Protocol documentation section with information about network messages mostly. You can also use other pages of this wiki, but be sure to check it's up to date.

Ultimately, the documentation you are looking for, as mentionned by the commenters, is Bitcoin Core's source code (unfortunately) ; if you want to create a full node implementation, you'll have to be bug for bug compatible.

Finally, to test your progress, you can use https://github.com/TheBlueMatt/test-scripts to see how your implementation handles some edge cases, as well as run it on testnet.