That's what the Ultimania's really are though. You get a few nice details here and there but for the most part, a lot of them are just glorified strategy guides with a few behind the scenes tidbits. I feel the closest you ever get to an actual "lore style" book is with the Anniversary FFVII Ultimania that released with the Compilation ten years ago.

There's also the stuff from the SNES guide books as well which details a crap ton of things that are never seen in the game, but considering all future Ultimania tends to ignore all of that info, it's safe to say it's likely non-canon. Funny enough, I find it's the non-FF games that tend to treat their Ultimania guides as actual lore books. Xenogears Perfect Works is probably the most famous example, but even the Chrono Cross and SaGa Frontier 2 Ultimania's have a surprising amount of story detail found in them that never made it into the games proper.

Still, it's nice to get some info and you do still learn a few interesting bits of info from these guides which is why people make a big deal out of them.