Main Legal requirements

Directive 2004/52/EC of the European Parliament and of the Council of 29 April 2004 on the interoperability of electronic road toll systems in the Community:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2004:200:0050:0057:EN:PDF

Directive (EU) 2019/520 of the European Parliament and of the Council of 19 March 2019 on the interoperability of electronic road toll systems and facilitating cross-border exchange of information on the failure to pay road fees in the Union:
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32019L0520&from=EN

Commission Decision 2009/750/EC of 6 October 2009 on the definition of the European Electronic Toll Service and its technical elements:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:268:0011:0029:EN:PDF

Other Legal requirements

Commission Implementing Regulation (EU) 2020/204 of 28 November 2019 on detailed obligations of European Electronic Toll Service providers, minimum content of the European Electronic Toll Service domain statement, electronic interfaces, requirements for interoperability constituents and repealing Decision 2009/750/EC:
https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=celex:32020R0204

Commission Delegated Regulation (EU) 2020/203 of 28 November 2019 on classification of vehicles, obligations of European Electonic Toll Service Users, requirements for interoperability constituents and minimum eligibility criteria for notified bodies:
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex:32020R0203

Directive 2007/2/EC of the European Parliament and of the Council of 14 March 2007 establishing an Infrastructure for Spatial Information in the European Community (Inspire):
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=OJ:L:2007:108:FULL&from=EN
http://inspire.jrc.ec.europa.eu/

ITS action plan: Communication from the Commission — Action plan for the deployment of intelligent transport systems in Europe (COM(2008) 886 final):
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:52008DC0886:EN:NOT

Directive 2010/40/EU of the European Parliament and of the Council of 7 July 2010 on the framework for the deployment of Intelligent Transport Systems in the field of road transport and for interfaces with other modes of transport:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:207:0001:0013:EN:PDF

Guide for the application of the directive on the interoperability of Electronic road Toll Systems (2011):
http://ec.europa.eu/transport/media/publications/doc/2011-eets-european-electronic-toll-service_en.pdf

Communication from the European Commission, Brussels, 30.8.2012 COM(2012) 474 final - Implementation of the European Electronic Toll Service:
http://www.parliament.bg/pub/ECD/122425COM_2012_474_EN_ACTE_f.pdf

EETS Registration

As reference is the EC web address with the list of links per MS:
https://ec.europa.eu/transport/sites/transport/files/national_eets_domains_registers.pdf 

EETS relevant studies:

State of the Art of Electronic Road Tolling  (MOVE/D3/2014-259):
http://ec.europa.eu/transport/modes/road/road_charging/doc/study-electronic-road-tolling.pdf

Technology options for the European Electronic Toll Services:
http://www.europarl.europa.eu/RegData/etudes/STUD/2014/529058/IPOL_STUD%282014%29529058_EN.pdf