EuroISDN is actually the name given to the Memorandum of Understanding (MoU) signed 1989 by several of the largest PSTN operators in Europe. It describes a common implemenation of ISDN. The ITU specifications leave a lot of implementation specific settings. Instead of each operator providing their own ISDN specification (or that of their switch supplier) equipment would interoperate all over europe. The standards are published by ETSI.
ISDN is a collection of signalling specifications and other telecom specifications. The basic signalling is specified in Q.921 (lower level) and Q.931 (call setup etc). Others include Q.956 (Advice Of Charge) which in turn references about ten other ITU standards.
The EuroISDN specifications based on these ITU specifications are ETSI ETS 300 403 01 for q.931 and several others and can be downloaded from www.etsi.org. Where these are based on an ITU specification only the differences are included in the ETSI specification.
To work on ISDN implementations, especially the more advanced features such as ROSE invokes etc you need a quite large set of ITU specifications, ETSI specifications and likely some from your PSTN operator as well.

ISDN Features:

ISDN Features

ISDN Specifications:

ITU Q.920
ITU Q.921
ITU Q.930 (6 pages) ISDN ISO-Layer 3 General Aspects (mostly irrelevant)
ITU Q.931 (125 pages) ISDN ISO-Layer 3 specification for basic call control
ITU Q.956.2 (51 pages) Advice of Charge (AOC) (refers to X.208, X.209, X.219, X.229)
ASN.1 description of Q.956.2
ITU X.208 (72 pages) Specification of Abstract Syntax Notation One (ASN.1)
ITU X.209 (25 pages) ASN.1 Protocol Specification
ITU X.219 (41 pages) Remote Operations (RO)
ITU X.229 (30 pages) RO Protocol Specification

ETSI 300 4003-1 Q.931 EuroISDN

Ericsson Guide to Understanding Telecommunications