You may also read Chapter 13 of [Kurosh's book](https://books.google.ru/books/about/Theory_of_Groups.html?id=B3lHYIuqPuQC&redir_esc=y). 
For instance, it contains a proof of Baer's theorem (cited by @Igor) which says that 

**all p-Sylow subgroups of a locally normal group are isomorphic.**

_Locally normal_ means periodic with finite conjugacy classes.