Producing secure software is challenging. The poor usability of security Application Programming Interfaces (APIs) makes this even harder. Many recommendations have been proposed to support developers by improving the usability of cryptography libraries; rooted in wider best practice guidance in software engineering and API design. In this SLR, we systematize knowledge regarding these recommendations. We identify and analyze 65 papers, offering 883 recommendations. Through thematic analysis, we identify 7 core ways to improve usability of APIs. most of the recommendations focus on helping API developers to construct and structure their code and make it more usable and easier for programmers to understand. there is less focus, however, on documentation, writing requirements, code quality assessment and the impact of organizational software development practices. By tracing and analyzing paper ancestry, we map how this knowledge becomes validated and translated over time. We find that very few API usability recommendations are empirically validated, and that recommendations specific to usable security APIs lag even further behind.