There are multiple programming languages that you could focus your efforts on and it will depend on which cryptocurrency you would like to use as a base platform for plug and play functionality. It would also be important for you to consider what the top programming languages are in 2015/2016, published by IEEE here, and TIOBE.
To establish a strong crypto programming foundation, I would suggest concentrating on first learning C++ by using a few beginner books (if you are on a tight budget, you can actually purchase many of these books from a nearby Goodwill or used book store for 1/20th of the price), and then begin to experiment with Node.JS, Python, Perl, and other relevant languages as these are typically compatible/accepted/supported in activate crypto environments.
For specifically programming with crypto technology, you can focus on C/C++ with OpenCL / Cuda for GPU programming. As there is a tremendous amount of capital and funding approved and dedicated now to crypto technology infrastructure and architecture (and rightfully so, brick foundation is crucial), you can secure a high ROI by focusing on those areas as well as wallets, mining services and rigs (C or C++). Also, you should definitely learn about the autonomy, decentralization, fault tolerance, and scalability of distributed hash tables (DHT):
A few great books I personally highly recommend you have in your library:
List of the major cryptocurrencies and base programming languages:
Bitcoin API and enterprise software platforms:
Once you are ready, you can explore the sandbox at Chain.com, which delivers the power of a “production-grade Chain Core in a simplified development environment, designed for rapid iteration” and it accepts/offers SDKs for: Ruby, Java, Node.JS and aside from having a unified interface, multi-language support, tutorials and sample code, and implementation guides, Chain offers a Virtual Hardware Security Module (VHSM); this is critically important because it is a device that protects and organizes digital keys for secure authentication and provides cryptoprocessing services. With the recent exploitation of security vulnerabilities, this is essential to any crypto platform.
Aside from Chain, there are many other Bitcoin APIs that you could learn about: Blockcypher, Block.io, Coinbase, and BitPay.
The Blockchain has inspired software that is already being tested and experimented with by enterprise technology companies for multiple commercial applications (albeit most of them being private Blockchain based, yet still effective). If you focus your efforts and time on tracking which applications will pick up the most momentum, and then making sure to become an expert in those programming languages, you should create a very competitive profile for yourself for multiple decades to come.
Recent commercial examples that are trending (some excerpts from wiki):
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