If it were me and I have been trying to get dcb to stop this habit for some time to no avail and have spoken with the parents about it...maybe give them a warning and explain that there may need to be some reimbursement for books and toys he has ruined due to the drooling and chewing on them. that would tick me off. I think you are doing everything right and I think 18 mos is old enough to not eat a book!! I have a 17 mon old in care who had this habit too but after a month or two of my e reminding him at about 14/15 mos he stopped doing it almost completely.
As far as biting friends go, this should be punished wioth a time out/loss of privilege and documented to show the often occurrence of it.
Maybe there should be a termination date threatened if the behaviour is not curbed. Say 3 weeks.