Below are some links to the best E-Books I know of on the internet which are free and comprehensive. I recommend emailing them to your local OfficeMax, Staples, or Office Depot copy center. Request them spiral bound, single sided, with a glossy cover and a vinyl backing. You’ll have an easy to reference book for under $20.
ALE, EDI, IDOC
Nobody does it better than Axel Angeli Here is his Cookbook for ALE, EDI, IDOC development
ALV Grid Control
How to Actually USE the ALV - great primer for users.
For ECC 6.0:An Easy Reference for ALV Grid Control
Smartforms
Swaroopa Vishwanath from Wipro Technologies put together this excellent tutorial on SmartForms which explains how to build a SmartForm and download the output as a .pdf
Cinthia Nazneen hosts this file which is the SAP official guide to smartforms in pdf format.
RFC, Parallel processing
This PDF by SAP Professional Journal explains the core 5 types of Remote Function calls, how to use them, and when to use them.
Others?
Please post comments with links to freely available pdf downloads. I moderate these posts and reserve the right to reject poorly written documents or documents hosted on sites I deem inappropriate. Please bookmark this page or subscribe to the RSS feed since I will be updating it as I find new resources.
If you are an author of one of these E-Books and would prefer I send people to a different link, please email me via the Contact page.

SCN Business card