According to the Wiki you can get cobwebs without silk touch using shears. Using a sword you need silk touch:
Cobwebs can be obtained by clipping them with shears. It will also drop itself if broken with a sword enchanted with Silk Touch.[fn 1][/sup] It will drop 1 piece of string if broken with a (non-Silk Touch) sword, if water touches or flows over it, or if a piston pushes it. It will drop nothing when broken using anything else.
Can anyone tell me why i can't put my silk touch book on my shears?
you don't need to because in 1.9 I Believe they added silk touch to shears by default ( Technically speaking, But you can't actually see the enchantment on the shears ) which means you can mine cobwebs by default without needing to put an enchantment on it.