Leather can also be gatherd from hunting or capturing Animals.
Leather requires no tech to unlock but the buildings that produce leather may need to be unlocked by a tech.
Leather is usualy a bottle neck in the early game since basic buildings like Houses require leather to be built and the gather rate of leather from buildings such as the Hunter are usualy to low. It is advisable to supplement you leather storage (and food storage) with hunting animal formations that are found on the world map.
Leather is also used by the Tannery to make Leathercloth. Leather is also used for the production of Bows. and in the production of Leathercloth at the Tannery or in the production of Bows at the Weapon maker.
Your main source of Leather in the early game will usualy be from hunters. but since hunting spots will disapear from your settlements after 800 population is reached you need to quickly switch over to animal farms.
(Note that gather rates vary due to state policies and resource avalability what is given here is the base gather rate of the hunter)