PLCs

Hi all,
could anybody suggest a good book to study about PLC ?
what are the different blocks in a PLC? Does it work on AC or DC supply?
when we draw a relay ladder diagram are the main parallel lines supposed to be connected to AC or +5V DC?

does a PLC use microcontroller or a microprocessor?

I think this is the best book about PLC's and by the way is free :

https://www.electro-tech-online.com/custompdfs/2003/08/plc_bookcp.pdf

It's worth to take a look at :

http://www.industrialtext.com/freestuff.htm
http://www.thelearningpit.com/

Bye!