Oecumenius

And he is said to have held in his hand a tiny open scroll [βιβλιδάριον]. Daniel, recalling such small scrolls [βιβλίων], says: "before him sat a judge, and scrolls [βίβλοι] were opened." (Dan. 7:10) The tiny scroll [βιβλιδάριον] was one in which were recorded the names and the multitude of the very impious who were to be punished. Therefore, it was also diminutively called a "tiny scroll" [βιβλιδάριον], since it is a kind of scroll [βίβλου]. …

Commentary on Revelation ↗

Bede

And he had in his hand a little book open. This is the book previously mentioned, which, long closed by its cover, was finally opened by the grace of the Lord, so that, according to the prophet, even the deaf might hear the words of the book (Isaiah XXIX). And his face deservedly shines like the sun, because he now carries the book open.

Commentary on Revelation ↗

Alcuin of York

And he had in his hand a little book open. The angel's hand is the working of our salvation, that is Christ's incarnation. The little book open is the declared grace of the New Testament. Note also that if the angel's face had not shone first, he would not be holding the little book open in his hand, because it is in his plan that the whole sum of the Scriptures was revealed. And he set his right foot upon the sea, and his left foot upon the earth. …

Commentary On Revelation ↗