I am wondering to know more about Baer groups and A-groups. I know their definition as below:
A Baer group is a group that all elements of prime power order have prime power index $[G:C_G(x)]$
An A-group is a group that all it's sylow subgroup are Abelian.
But I want to know more a bout them. For example are these groups always nilpotent? If someone introduses a book that can help me, I will really appreciate him/her.