Using CiviCRM for Library Management
if you are using Drupal - and you aren't needing to use clever "enter the book number and have all the details pulled in from external software" when adding a new item to your library, then i could see a relatively easy path of making each item a node, using unlimited group of fields to track 'out date, in date, person'- and use civicrm entity 'contact reference field' to track which civi contact has the item

and do the rest with views and webforms

there is also https://www.drupal.org/project/library but it may be as much effort as building from scratch imo