Parchment is doable, if you put in the effort. Its just you have to use flux stone to make quicklime through a 3 step process(flux stone -> quicklime(kiln) - > milk of lime(ashery)), then use it on a recently butchered hide before it gets tanned. And you also pretty much need to do the scroll route, since you can't bind books. You can turn the parchment into quires, and they do seem to use it, but it is my understanding that quires that never become books are somehow lacking, but I have no evidence to back that.
Caravans will typically bring 10-20 sheets if you ask for them, and about 10 codexs, so that will be a very slow build. If you just want to max out bookcount without making them, make a tavern and then murder the guests. Poets often arrive carrying 1-3 books and as far as I can tell there are no real repercussions to mass murder of guests.
Also, the secrets of life and death can only be orignally written in worldgen, you can't write them yourself in a fort. However, if you fort gets attacked by a necromancer, he may carry his book of secrets, which you can acquire, and while your scholars can't originate the secrets, they can copy them and a copy of the secrets works just fine. You can also retire a fort, play as adventure to get secrets, travel to fort, retire or otherwise deposit the secrets, and then reactive the fort to gain secrets.