Introduction
https://en.wikipedia.org/wiki/Electron_microscope
The above links contain the basic information about electron microscope. This material is easy to follow.
https://www.jic.ac.uk/microscopy/intro_EM.html
The above links contain the basic information about electron microscope. Here it is somewhat advanced.
http://www.fei.com/introduction-to-electron-microscopy/
The above links contain the basic information about electron microscope. Here you can get in depth knowledge about electron microscope.