The Handbook of Open, Distance and Digital Education, a great (1400+ pages!) resource on elearning has been released as an openly licensed book. Great essays in here on learning theories - I will write more as I get through this, but it looks some of the most interesting writers/researchers on elearning are well represented here. Pay a lot for the print copy or download it free as a pdf. https://link.springer.com/referencework/10.1007/978-981-19-2080-6