Great Mexican cookbooks -- that one is easy. The authors to look for are Diana Kennedy and Rick Bayless. I do not like Bayless, myself, for complicated and totally personal to me reasons I won't get into, but there's no question that he's talented, knowledgeable, and serious about writing usable cookbooks. Diana Kennedy is wonderful and I adore her.
Fajitas like your local Mexican place makes are a very tricky thing. I'm no expert on the various Mexican cuisines, but my sense is that fajitas as most of us know them are basically a TexMex/Southwest thing. That's not a criticism, it just means that digging into Kennedy or Bayless may not produce the results you want. Unfortunately, I haven't the foggiest idea whether there are truly great TexMex cookbooks out there -- probably, but I just don't know. Hopefully somebody here does.
What I can suggest is two approaches.
First approach: if the place is not a corporate-run thing (a local chain, competing against Chili's or whatever), but more of a local place, try to eat there often, leave biggish tips, be cheerful and friendly, and chat up (not in a sexy sense) the waitstaff, especially any of them who appear to be more longstanding rather than recently-hired help. What you're trying to do is get to be known to the owner/manager, and thus to the chef. From there, what you do is just ask what they put in it. You'd be surprised: lots of people are happy to just tell you.
Second approach: if that's not your bag, or it's not going to work, or whatever, get one extra order of fajitas and take it home. Rinse your mouth out so it's at baseline and really taste. Think about it. You know what grilled beef or whatever tastes like. Is it grilled? Probably, but maybe not. You know there's salt in there, probably pepper. What else? Cumin? Cilantro? Cinnamon? Clove? Dried chile (if you don't know this, you need to buy some dried ancho peppers, toast one gently in a pan, and nibble at the skin --- skip the seeds --- the flavor is distinctive and you'll need to know it)? Okay, now where are you stuck? I'd bet you're stuck on four things above all: a peculiar dried chile flavor you can't quite place (they're hard), whether there is tomato or not, a remarkably fuller, richer flavor than you seem to get when you try it, and then some indefinable odd something you just can't quite put your finger on but seems weirdly familiar.
My bets: 1) they may well be using New Mexico dried peppers, because they're very cheap in bulk and have excellent flavor without a lot of heat; 2) yes, there is probably tomato in there, but it might well be tomato paste; 3) add a generous pinch of sugar early in the process and cook in a healthy amount of fat at every possible opportunity; and 4) that's acid you're tasting, cooked --- try lime juice, white vinegar, cider vinegar, or rice vinegar, right at the start of the process so its easily identifiable flavors fade.