Books on sources of Islamic Law are many but below are the names of five great references on usul al-fiqh that should be helpful to you.
–Irshad al-Fuhul by al-Shawkani
–Nuzhat al-khatir Sharh Rawdat al-Nazir by Ibn Badran
Read Also:
–Al-Mustasfa min `Ilm al-Usul by Abu Hamid al-Ghazali
–Sharh al-Kawkab al-Munir by Ibn al-Najjar al-Futuhi
–Ihkam al-Ahkam by Sayfu al-Din al-Amidi