I've been looking for the official Graduale Romanorum which sanctions which hymns, chants, etc. are allowed to be performed during Mass.
Unfortunately, I could only find the Latin version in digital format.
Also, if anyone knows a good hymnal in Latin or Croatian or where I can find official documents with texts, it would be much appreciated.
This comment on this article inspired me to look for it.
The Graduale is officially published by the Solesmes Abbey, in France. Here is the shop website, with some examples of chant books, including the Graduale Romanum. As Ken said, you can find it in some benedictine monasteries. At least I did.
There are some online full texts around (e.g. here), but as far as I know, not the most recent edition (if it were so, it would probably be a copyright breach to have it, and surely a sin).
There are plenty of more resources online with chants. For example, here, here and here.
Last but not least, there is this impressive website with links to videos of chants, with scores and lyrics, for pretty much every time of the year.
Check out Benedictine monasteries in your region or other institutes of religious formation, especially for Croatian sources. Then again their is always Amazon.
Addendum:
The Benedictine Order in general and Solemnes Monastery in particular have been asked by Rome to preserve the heritage of Gregorian Chant as the official Sacred Music of the Church in the Latin Rite.
A PDF scan of the 1961 Graduale Romanum is available from the Church Music Association of America (CMAA). So is the 1961 Liber Usualis. (cf. CMAA's Music PDFs and Book PDFs)
GregoBase is a comprehensive database of Gregorian chants.
CC Watershed has scores, audio, and videos of Mass propers.
