WARNING : NUMBER ! DON'T READ IF YOU ARE SCARED OF THEM
(even if it's so easy to do it that it shouldn't even called math at this point)
There is a Tl;DR too.
nurse have 1.25% chance to drop the beaker overall.
If something drop medicine, it have 5.37% chance to drop rare medicine.
Rare medicine have 0.62% chance to drop a beaker.
That's 0.033% chance per drop of medicine.
As you can see, you better go loot a nurse, because on the other case you won't get it (beside maybe health crate or pop n pill, but even so nurse is the way to go).
I found a few already, but unless you actively hunt the nurse it's likely you won't find one. Rare med are *really* rare.
Book are basically about the same problem.
Tier 1 book are found in a loot that is actually everywhere, and in some building a hight quantity. There is little tier 1 book, so the prob of draw all of them isn't low. In the end, tier 1 book can be found super quickly.
However, after that bookcase are useless.
tier 2 book are found in safe (wall safe, safe, gun safe). Safe are more rare than bookcase, there is less of it, are locked behind iron (so there is a big chunk of your time you won't be able to loot them). There is also way more book in tier 2.
The prob to find *all* tier 2 book is rather low because of the hight number of book (tip : the more item you add the worst it become really quickly, as nothing stop you from drawing the same book again and again). As safe are rare AND long to loot AND don't even bother drop a book in first place often (25.97% for gun safe, 26.31% for wall/desk safe), it's extremely long to get them.
And by extremely long I mean either you are lucky and spend *all* your time hunt for book, or either you will NEVER find them all even if you are in a team of 5 players in a decent period of time (=server reset before you get it).
Tier three is another thing. It require either steel, only 27.12% chance in munition box, 66% on a feral. I'll pass on the fact that steel is gated in a way that make it nearly impossible to get (see the very end if you want to read about it), the chance to have them all would be reasonable only if we could have a chance in first place to loot them.
Feral come in two way : screamer during day (sometime), and 7d horde. Even so that's not even one per screamer and basically maybe 2-3 per 7d horde if you do good. You need to loot them too, so you are *forced* to make a tricky defense that allow you to loot them, kill them (not exactly the most easy thing to do; ranged weapon are costly in ammo, but at last there is way in melee).
TL;DR
Remove some book of tier 2 & 3 that really don't fit here (like remove vent block in tier 3) and move them in tier 1
Increase the chance to get tier 2 by a good amount, and same for tier 3 ammo box (only ammo box, they are rare, make them worth it)
Just increase the prob isn't enough here, need to play with both number of loot, prob of other loot too. Because it's just way too low.
*on a last note, about steel. If you intend to have player abuse the xp system yes steel is doable. Otherwise in a normal gameplay you can't really get it.
You can assume someone would eventually get 100 wood/iron/stone. Maybe 50 engineering, as a LOT (too much) perk require lv50 engineering. Misc would not be done by them and still get some xp, same for athletic maybe.
But beside this, you don't get any xp when hunting, killing, looting (always been super low in vanilla too), etc. And thus near lv50 won't progress much.
There is way for a player to grind active skill by abusing the experience system. But there is no real fix for this, as it's because of a lot of the mechanic here (hp per block, hp per zombie, buff killing zombie, lower overall level and less quick craft perk and speed bonus from skill).
Eventually you can ignore this and call it fine, but don't expect people to get any of the end game atm.
Meanwhile if you want to help it you could try to :
Make a way to grab decent amount of xp from medicine and stuff like this.
Cloth, light, heavy armor xp : beside grind it, nothing happen. Could eventually have something done with those skill so we get some xp out of it (like awning block is misc and could be tailoring, even if this very change won't help the player level ...)
Removal of weap skill mean removal of a lot of xp you can eventually get. It's a good idea, but we could use a bit lower player level experience requirement to deal with it.
Harvesting isn't used anymore because of the farming/fertilizer system. Same as weapon skill, if we can't use it, lower a slight the player experience requirement. Unless next version could work.
Zombie loot nothing, like nothing. We don't ask much : maybe a bit of scrap, dunno. Meanwhile, as they loot nothing, we don't kill them. As we don't kill them, we don't use all the active skill (blunt archery etc). There are already super low in first place (weapon buff system) but here, if there is no point in using it anyway (most dmg come from buff so clearly make no difference to level your skill) that's a lot of experience that people won't bother get.
Hunting could get a boost for harvesting carcass, and get more xp ... atm, it's low, and the lack of use for knife beside hunting doesn't help (low dura and range so can't use it much in fight).