Honestly, most of the Manual is design stuff... the stuff you need, AISC 303 (Code of Standard Practice); AISC 360-10, (Specification for Structural Steel Buildings); and RCSC Specification, are free downloads on AISC website.
I agree with Duke. You can probably get them cheaper, but, you really don't even need them it doesn't sound like. If you know someone who is a member they can get them at 50% off for you.
As an inspector, if I came to your shop to look at the shop fab of some light weight structural steel I would be more likely to appreciate your having a current copy of D1.1, 1.3, and 1.4 on the shelf and having at least a basic understanding of how it applies you your work.
Personally, I think every shop should send an employee through the CWI Seminar. Even if they don't take the exam. Just the experience of the week of study and practical application would open a lot of eyes as to the application of the codes to each one's individual needs.
But, if you already have the AWS codes and want to add another book, that is a good one to have on the shelf. Just don't leave it on the shelf...CHECK out what it says.
I have both the 13th and 14th editions. I like having the hard copies and not just the downloads. But, I'm an inspector and use them regularly and have to show fabricators things where I can't always carry my laptop out on the shop floor or up to the 10th floor of the building.
Have a Great Day, Brent