The Jeffus book is good...but I find that he gets bogged down in non critical discussions and forgets teaching the basic principles of welding. Yes, the exercises are good, but people need tips and tricks and what to watch out for if they are a beginner. That book is designed for someone with an instructor around to give the core part of instruction, imho...
For basic information for welding, Haynes puts out a decent primer. It's not advanced and is in laymen's terms. But it is chock full of the does and don'ts of welding and gives a decent overview and detail of actually "How to". So many welding publications give little more than theory and don't discuss actual technique. I won't post a link directly to some free available copies in pdf (easily done with a google search) in order to prevent a conflict with copyright issues and this site...though I suspect in the older editions with essentially the same information, it may have expired.
I also found an old Military welding manual (Technical Manual and Instruction Guide: Welding Theory and Application, War Department 3 June 1943, Pub.# TM9-2852) that is basic and to the point written in WWII about principles of gas and stick welding. I've not found anything to compare in basics and explaining detail like it. It's a little outdated in technology, but written in plain language that so many manuals and instructional books were back then. Nowadays, just as Jeffus' book is or has become through many redos and editions, books/manuals gets bogged down in the language and just don't communicate simply or plainly anymore
Here is one on Amazon:
Welding Theory and Application: Technical Manual & Instruction Guide TM 9-2852: War Department: Amazon.com: Books
I also have an older one from the 70's which incorporates most everything Jeffus book does but in a more straight forward approach and language. It's out of print, but I think there are new, old copies around. It is: Welding Technology, 2nd edition (don't think there ever was a 3rd) by Giachino/Weeks/Johnson published by American Technical Publishers. I think ATP became AAVIM.
I found it here:
Welding technology - Joseph William Giachino, William R. Weeks, George Stewart Johnson - Google Books