The book is "Now I Walk in Beauty", edited by Wilko Brouwers: not a hymnal, but a children's classroom songbook to provide supplementary material for lessons based on the Ward method. It starts with the simplest melodies, using just two notes (do, re) and builds from there, adding new skills step by step through the 100 songs in the book. (A table in the back of the book spells out how new musical features and corresponding notation are introduced at each point.)

About 40 of the pieces in the book have religious texts; some are classic hymns and Christmas carols in English or Latin; some are short but authentic melodies from Gregorian chant, such as antiphons from the Office; a few are musical exercises and simple original songs composed by the editor. The remainder of the pieces include folk songs from around the world, play songs, and melodies without words.

It all builds up to song #100, "Stella splendens", a pilgrim song for Our Lady of Montserrat, in Latin; it adds two new musical features, as the song features a key change between its refrain and its verse; and it is a choral piece for two vocal parts.

The book is available from the CMAA Shop at
https://shop.musicasacra.com/product/now-i-walk-in-beauty-coilbound-wilko-brouwers/

That's the one. Thanks.

It's intended to be used by each child. (The single-copy price is on the CMAA shop site, but volume discounts are available.)