This is the most common "Usuula" book, focusing on the three questions asked in the grave: Who is your Lord? What is your religion? Who is your Prophet?
Many universities with Islamic Studies departments (Al-Azhar, Islamic University of Medina, University of Jordan) host open-access PDFs of classical Usul texts. Search for "[University Name] Digital Repository Usul al-Fiqh PDF". kitaaba usuula pdf