Any documentation available that would help developing a Bitcoin Client?
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.