75 ILCS 16/40-25
If the trustees deem it best, in order to provide and secure the necessary money to do any or all of the things they are authorized to do in this Article, they may, at any time, borrow money and execute a mortgage on a library building or site owned or being purchased or on a library building being constructed for the district for an amount not exceeding 75% of the value of the building or site as improved by the plan. The money so obtained shall be used exclusively for purchasing a site or a building, constructing a building for library purposes, remodeling, repairing, or improving an existing library building, building an addition to an existing library building, erecting a new library building, or purchasing necessary equipment for the library, as provided in the plan. The trustees shall determine and certify, by ordinance, the amount of the mortgage and the amount of principal and interest to be retired each year for a specified number of years. The board shall, in each succeeding annual appropriation ordinance, include the amount so certified and shall, for the amount so certified, include that amount in the annual library tax levy or levy (in addition to the other library taxes for the district) a special tax to pay that amount.
75 ILCS 16/40-25