Within the Law Library you will find useful titles in the legal philosophy collection - shelf marks begin Jurisp or KA. However, as constitutional law is a taught subject, look out for SOLO results directing you to the Law Reserve Collection.
lndeed, it may be that SOLO will also reveal some useful sounding titles in the Bodleian Social Science Library - this is in the neighbouring building, a short walk away.
Subject searches to use in SOLO include:
Constitutional law - theory
Constitutionalism and democracy
Law - political aspects