This thesis presents the results of usability testing of the eZaKi application, which enables users of the Zagreb City Libraries to manage their membership, search and borrow library materials. The aim of the research is to test the usability and user experience of the eZaKi application, which was developed by the Zagreb City Libraries in cooperation with the company VIVA info d.o.o., while the purpose of the work is to identify the difficulties and challenges that users encounter while performing tasks and to develop guidelines for improving the user experience. The research was conducted using the Loop11 tool, which is intended for user experience research, with the participation of six respondents who regularly use the eZaKi application. The results showed a high rate of inability to find the eZaKi application on the homepage of the Zagreb City Libraries (www.kgz.hr) and significant problems in its use for borrowing e-books. Based on the experiences of the respondents, the following guidelines were proposed for improving the eZaKi application: interface integration and design improvement, improvement of the application's statistical functions, better promotion of the application so that users are familiar with its capabilities, clear explanations of the application's functions to reduce user confusion, integration with other services of the City of Zagreb Libraries (for example, the "Ask Librarians" service), and consideration of the integration of the eZaKi application with the Kindle platform. This research contributes to the Library and its staff, the registration office and the development team to improve the operation of the application and provide users with quality service