I'd recommend against either one of those. The wiring diagrams alone are a pain to deal with (too generic usually) and the info is usually just summarized out of the GM manual.
Helms sells them, usually pricey, but you can as mentioned check ebay for the manuals. Make sure it's not for the S10 though, as they were considered "light duty trucks" just like the full-sizes, and the manuals cover/title is very similar. (or at least were early on in the 80's)