Pretty hard to go wrong studying on your own (if you choose the right material), but I will chime in that you needn't sweat it for the first solo. Don't let your extra reading distract you from what you know you will have to do for that first solo (which is really very simple).
At this point in your training, I think good debriefings with your CFI are way more relevant than "ground school" as in "classroom stuff".
Fact of the matter is, a good CFI will only sign off a student for solo if that student has effectively
already made a few circuits on their own, in the sense that the instructor didn't have to touch anything or say anything. I'm not an instructor, but if I was, that would be one of my requirements.
Whether or not that is the case with you and your CFI, he definitely wouldn't sign you off if you didn't just seem ready for the simple practical exercise of the first solo hops, where the theory and regs (and most other ground stuff) are not going to be of much use anyway.
FWIW, to prepare yourself specifically for the solo, I'd recommend chair flying. Just sit and mentally go through a series of takeoffs, patterns, landings... say everything aloud, and use whatever written checklist you've been using. Picture the whole thing in your mind, in real time. Picture each solo landing just like the best landings you've made so far (there must be a few really good ones, yes?)
Also review your emergency procedures, and runway and taxiway markings (as they apply to the airport you will solo at). Also ask yourself things like "if the wind changes up and I have to land the other way or on another runway, what do I do?" If you're not sure about anything like that, ask the instructor. Take it from me, you don't want to wait for him to tell you... because he might forget to.
And most of all, remember that you've basically already done the solo, with the CFI just sort of along for the ride. When you take off, you'll feel real nervous glancing over at that empty seat, but I can almost guarantee that after the first landing, as long as the airplane is still flyable, you'll be thinking "what do I need that clown for? I'm in
command!!"
It's a great feeling, a HUGE confidence-builder, and you don't have to be some sort of "natural-born flyer" to experience it.
Just don't get carried away with that feeling, and you'll do fine.
Good luck!