PLC Programming Handbooks
The Logix5000 Essential Manuals
The Allen Bradley Logix5000 family (ControlLogix, CompactLogix, FlexLogix, SoftLogix) has some very good manuals. If you are just starting out or need a refresher here are the key manuals and the order I would read them. If you have RSLogix 50000 installed then you will find some of these in the Help > Online Books menu. Revision 16 also has some great videos in the Learning Center.
For starters there is the Quick Start manual.
If you are getting into programming and designing a system then you'll want to start off with the Common Procedure Manual. It has a lot of helpful examples dealing with all aspects of the system.
Next comes the nitty gritty of each instruction. It's a good idea to at least peruse all the instructions so you have an idea of what is available.
An often overlooked manual but filled with great information for getting the most out your designs is the Design Considerations Reference Manual. Certainly a must read if you are knee deep in the development and programming of Allen Bradley PLCs.
Specifics for the hardware can be found in the User Manuals and Installation Instructions for the PLC.
If you are deciding on which network to use then see the Design Considerations manual and the section "Determine the Appropriate Network". Otherwise, for existing networks the following are helpful.
The links are from the AB site so they are the latest and greatest manuals. Let me know if any of them are broken or if I forgot one you think is essential.