SEPA Instant rulebook is available on the European Payments Council website.
Detailed documentation including ISO20022 field level specification is available on the European Automated Clearing House Association (EACHA) website.
The European Central Bank provides Documents on TARGET Instant Payments Settlement (TIPS).
The rulebooks are not available for the public, neither any technical specifications at the moment. Participating banks has access, and their vendors could access the information via their partners.
The central bank of Hungary (MNB) has publications available in Hungarian: