Even if you find a spell book with flame tongue (let's say) near the end of D1 when you started as an AK with maces, it is almost certainly still going to weaken your character to suddenly switch away from training maces and instead try to get flame tongue online. Basically it is the same problem you talk about when you find a book at char level 5 or 6. The game is pretty tough right from D1, and you would usually be putting yourself at a significant disadvantage by choosing not to develop the primary method of killing that is provided by your background. (Obviously some backgrounds play a bit differently here, and species choice can influence it, but in general.)
Because further skill levels get more expensive, it might actually make sense for non-book backgrounds to get a useful low level spell somewhat early (say around character level 8 or 9), sometimes. Apportation, animate skeleton, maybe even repel missiles. But I can't think of any time a non-book background would want to pick up spells earlier than that, even if they were readily available. (Okay, old animate skeleton would be worth it. Not sure if new animate skeleton would.)
So, in effect, guaranteed books spawning on D1 to D3 would introduce another weird special case that would primarily serve to give book backgrounds access to more spells, right off the bat. I don't think book backgrounds need that buff, and it would seem to diminish the impact of one's choice of background and (later) god. Your background gives your character its starting point. Why should there be guaranteed drops that are basically like, "well, here's a less powerful version of stuff you would have gotten if you had chosen a different background"? Should a blowgun and a few poisoned needles be a guaranteed drop on D1 to D3? Or a wand of random effects? A single throwing net? A -2 quarterstaff?
Berder wrote:It's easy to do a melee start with any background since starting weapons are all over the place.
Yes, with any background you
can pick up the first weapon you come across and start using that as your primary method to kill stuff, but that's very rarely what you
should do.
The "warrior" backgrounds (Fi, Gl, Mo if you are Ghoul or Troll, and Hu) all start with better weapons than you are likely to find in the early game, and the "mage" backgrounds should be killing with spells. The "adventurer" and "warrior-mage" backgrounds are a bit special, and obviously some species can play differently—troll wizard will mostly kill with its claws. But yeah, you basically want to be using your background "kit" to kill stuff in the early game. For all backgrounds. That's why background choice matters, even though it doesn't dictate how your character is going to end up, which I think is a good place for backgrounds to be, in terms of design.