Would you please explain the policy on overdue books?
The answer to your question is complex because it can vary depending on the library and your affiliation. You can access the Fines page for information about the Harvard College Library which includes Widener and Lamont.
Generally, the fine for overdue books is 50 cents a day. However there is a 3-day grace period (for all non-recalled items). For overdue recalled items, the fine is $2 a day with no grace period.
The lost fee is $110, but if the book is returned, the lost fee is waived and only overdue fines are incurred.
There are several instances in various Harvard libraries where there's reserves, non-book formats and other circumstances which have 1,3 or 5 day loans or even hourly check-outs and in those situations, you would have to ask at that individual library about their fines.