The bests tutorials are provided by Jeff Potts in his website ecmarchitect: Alfresco Developer Tutorial Series
Personally, I learned using Alfresco SDK from these tutorials. The series uses a single business case as an example that covers many different technical aspects of Alfresco. Each tutorial takes you through hands-on customizations, step-by-step, and includes ready-to-build source code so you can follow along using your own local installation.
Also, you can take a look at Jeff Potts's book Alfresco One 5.x Developer's Guide - Second Edition