This book was literally full of spiders
This novel is called This Book is Full of Spiders by Jason Pargin. It gives an insight into the adventures of two men who take a mysterious drug and develop the ability to see the supernatural.
And yes, this version had actual spiders in it. At least the story involves spiders, but we are not sure if every copy comes with its own eight-legged creature. You can ask your local bookshop about it.