In addition to the subject searches mentioned above, in this area it may be worth trying other subject searches in SOLO such as
World Trade Organization -- Rules and practice and/or Foreign trade regulation and/or International trade
The Law of Subsidies under the GATT/WTO System by 
WTO Agreement on Subsidies and Countervailing Measures : A Commentary by 
WTO Disciplines on Subsidies and Countervailing Measures by The EC’s State aid rules apply to any subsidies granted by the Member States of the EU
Finding commentary on SOLO
Subject searches to use in SOLO to find commentary in books (both e- and print)
Subsidies Law and legislation + European Union countries
Government aid -- Law and legislation + European Union countries
Economic assistance, European -- Law and legislation
Subsidies - European Union countries
European State Aid Law and Policy by 
European Union Law of State Aid by