What do you call someone who ornaments the letters and pages of books [closed]

The decorated initial letters or marginal illustrations which can be seen in some manuscripts are called illumination, and those who do them are called illuminators. There's an illustration below, but it's not simply a thing of the past.

From Lexico:

illumination

2.1 [count noun] An illuminated design in a manuscript.

illuminate

3 Decorate (a page or letter in a manuscript) by hand with gold, silver, or coloured designs.

‘this is a carefully written, large-format manuscript illuminated by Leonardo Bellini’

Detail of a leaf from a Gradual: Initial P with the Nativity; 1495; ink, tempera and gold on vellum; each leaf: 59.8 x 4.1 cm; Cleveland Museum of Art via Wikipedia

Although we usually call the person who does the artwork for a book an illustrator, embellisher can be used for artwork and other flourishes, the choice/design of decorative type, etc. We can say, for example, She embellished the book with her illustrations, although you can embellish a book with someone else's illustrations as well.

illustrator

a person who draws or creates pictures for magazines, books, advertising, etc. Oxford

embellisher

He who or that which beautifies or adorns. OED

embellish
To make beautiful with ornamentation : DECORATE

a book embellished with illustrations m-w

If something is embellished with decorative features or patterns, it has those features or patterns on it and they make it look more attractive.

The boat was embellished with carvings in red and blue.
Ivy leaves embellish the front of the dresser. Collins

... payment of 301. to Gideon Royer, the king's writer, flourisher and embellisher, for writing, flourishing and embellishing three skins in vellum.. D. P. Dukes; Stuarts and Romanovs p.134

Also, he embellished the books with many lively sketches of scenery, Indians, animals, fish, canoes, and ships, along with dozens of maps and charts which were remarkably accurate for that day. ... S. E. Morison; Best Sellers from the U.S. Government Printing Office.

Comment upon the charming specimens of Fowler's skill as a book embellisher and decorator which appear in this issue of THE BOOKMAKER... American Printer and Lithographer.