user41767: 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

user3056: I'd suggest you to start from making an offline blockchain-parser.
Use the block stored on disk and start parsing them in order to build (and update) the UTXO database.
This is the most crucial and performance critical part - if you have it working, the network part will not need much more of the resources.
Later, when you start playing with the network part, you will likely encounter problems that are much harder to analyse and fix, because to the asynchronous matter of the events.
Still having a stable (and well performing) blockchain parser will benefit you hugely at this stage.
Also I'd advise you to not implement memory pool and transaction relaying from the start - make sure that processing of new blocks is stable before adding the memory pool feature.

user41703: 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.

41767: <p>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?</p>

<p>EDIT:
I'm aiming for a full node with wallet capabilities</p>
 3056: <p>I'd suggest you to start from making an offline blockchain-parser.
Use the block stored on disk and start parsing them in order to build (and update) the UTXO database.
This is the most crucial and performance critical part - if you have it working, the network part will not need much more of the resources.</p>

<p>Later, when you start playing with the network part, you will likely encounter problems that are much harder to analyse and fix, because to the asynchronous matter of the events.
Still having a stable (and well performing) blockchain parser will benefit you hugely at this stage.</p>

<p>Also I'd advise you to not implement memory pool and transaction relaying from the start - make sure that processing of new blocks is stable before adding the memory pool feature.</p>
 41703: <p>The bitcoin.it wiki contains a <a href="https://en.bitcoin.it/wiki/Protocol_documentation" rel="nofollow">Protocol documentation</a> 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.</p>

<p>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.</p>

<p>Finally, to test your progress, you can use <a href="https://github.com/TheBlueMatt/test-scripts" rel="nofollow">https://github.com/TheBlueMatt/test-scripts</a> to see how your implementation handles some edge cases, as well as run it on testnet.</p>
