I am working on a supplement that I will publish under the OGL 1.0a. In one part of my supplement, I list a number of OGL-works as recommended further reading - however, I do not use any rules or mechanics from them in my work. Do they need to appear in my Section 15 citations in my OGL statement? (Along with copies of each of their own Section 15 citations?) Or does just mentioning/recommending a book not constitute usage? I looked at the definition of "Usage" in Section 1 but was not sure.
What you're describing is Nominative Use — a use that involves only naming a thing.
The OGL doesn't control or limit nominative use at all — because the name a book wants to be called by others isn't content, it doesn't fall under the OGL.
Nominative use isn't controlled by copyright either — it's long recognised that the name used to tell other people about a thing exists to be used, freely and without limit, so using the name for its intended purpose isn't inappropriate "copying" of someone's creative work. (Fair Use doesn't enter into it either, since Fair Use is about stuff that would normally be controlled by copyright, but that it makes sense to create exceptions for. Nominative use is a different, specific "no that's not even about copyright law at all" thing separate from copyright and its Fair Use doctrine. At best it touches on trademark law, but trademarks aren't an issue unless one is being used for deceptive marketing.)
To put it another way: without me claiming to be a lawyer, I can still say confidently that nobody gets in copyright or OGL trouble for nominative use, especially not "hey, these other books are great and you should buy them."
I have it from a very good OGL authority that I do NOT need to cite such works in the OGL statement. They said that basically, just mentioning a work does not meet the definition of the "use" of open content, as it is defined in section 6 (the piece that drives what you need to put into section 15.) So the answer is "no." I'm still leaving JP Chapleau's answer checked because technically it is correct and he answered first.
You could do it that way, but shouldn't.
Mere reference to a work is almost certainly not a "use" of that work. Assuming you have some corner case more extreme than I can think of (maybe in advertising materials?), and that corner case does constitute use, it is even harder to imagine a situation where that use wouldn't obviously be fair use, and so simpler than using the OGL.
But, if you were referencing that material in a way which might constitute "use" (unlikely), and also were unwilling to rely on fair use (unlikely and weird), you would then need to have some license for that use. In this weird, doubly-unlikely case, OGL is the obvious license, and if you use material under the OGL license, you would need to list it in Section 15.