I suggest looking over some photography tutorials on YouTube. While Iray is a bitch and real world values don't apply to it, specifically for lights and fstop and how it affects DoF, if you look past the specific values they use there's a lot to learn from photography. From lighting setups to tricks to cameras and how they work, to even color theory and importantly, composition.
Past that, there are plugins people have made to have a more realistic approach on how real camera's work, IDG's photography series are awesome for that with a rework on the iray camera to be more realistic in values to even some awesome models and materials for diffusers, reflectors and grids.
Besides that, you could always ask here. There's a few people that can help either for theory or practices, or even more specifically about iray.
