I have worked with both HFSS and CST and believe that for a beginner CST is more user friendly. CST also commonly implies less computational time.
From my experience both present similar simulation results, considering the mesh cells are well defined.
Regarding introductory books, just look at the documentation available in the software. You can find information there about the solvers and how to setup your design.
You also have several examples in both softwares you can use to practice.