List of external resources offering valuable information around bitcoin development.

Implement

Find detailed information about the Bitcoin protocol and related specifications. https://bitcoin.org/en/developer-guide

Learn

Bitcoin Edge is an initiative to aid the Bitcoin ecosystem to scale "at the edge", this means to scale the development capacity, expertise of the industry and Bitcoin adoption.
https://bitcoinedge.org/

Program

Learn how to program a bitcoin library with this hands-on guide from one of the leading teachers on bitcoin and bitcoin programming. Author Jimmy Song shows you the basics, including the math, blocks, network, and transactions behind this popular cryptocurrency and its blockchain payment system. You’ll also learn how simplified payment verification and how proof-of-work works.
http://programmingbitcoin.com/

Mastering

An in-depth technical guide to how bitcoin works, for developers and techies.
https://antonopoulos.com/books/

Scaling

Providing the Bitcoin development community with a physical space where software developers and researchers from all around the world, working on projects related to Bitcoin, can meet to discuss the challenges and solutions related to Bitcoin scalability.
https://scalingbitcoin.org/

Contribute

Bitcoin is an experimental digital currency that enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate with no central authority: managing transactions and issuing money are carried out collectively by the network. Bitcoin Core is the name of open source software which enables the use of this currency.

https://github.com/bitcoin/bitcoin/

Improve

BIP stands for Bitcoin Improvement Proposal. A BIP is a design document providing information to the Bitcoin community, or describing a new feature for Bitcoin or its processes or environment. The BIP should provide a concise technical specification of the feature and a rationale for the feature.

https://github.com/bitcoin/bips

Community

A reddit community focussed on developer related news and info around bitcoin.

https://www.reddit.com/r/bitcoindevnetwork

Wiki

The bitcoin wiki.

https://bitcoin.it

Questions

Bitcoin Stack Exchange is a question and answer site for Bitcoin crypto-currency enthusiasts.

https://bitcoin.stackexchange.com/

Extensive list of Resources

Bitcoin is a revolutionary system that is quite complex and has a steep learning curve. Make sure you have a decent grasp of the system before you store a significant amount of value in it. Note that this list is open source; please suggest additions on Github!

https://bitcoin.page/