If you're interested in something that's not strictly real-time a good book is physically based rendering http://www.pbrt.org/ . It uses a literate-programming style which is neat. It's about path-tracing which is similar to ray-tracing but can give an unbiased approximation to the rendering equation (which is a reasonably accurate model of "everyday" optics.) Here is a JS+WebGL (mostly WebGL ;) ) interactive path-tracer: http://madebyevan.com/webgl-path-tracing/ --- Vulkan is pretty boring to be honest. I'm very doubtful that you want to bother with it if you're just starting. Using these APIs is like filling out tax forms, but Vulkan is a lot more effort than OpenGL (but, OpenGL is weird...) Learning shaders in OpenGL will be pretty transferable knowledge. Vulkan uses a lower-level assembly-like language, SPIR-V that doesn't pretend to be C (like GLSL) but there are GLSL to SPIR-V compilers. I can't comment on DX (I haven't used it.)…
Also recommended in this reply