10:2 and he had in his hand a little book which was open. And he placed his right foot on the sea and his left on the land;

and he had in his hand a LITTLE BOOK which was OPEN.

The LITTLE BOOK has caused much debate. Some scholars believe it is the same one as was sealed with the seven seals (Revelation 5:1). Others feels it is a booklet or even a part of the book of life (Revelation 20:15), since later (verse 10) it is consumed just as life exists INSIDE of us.

Apparently the contents are visible to John. That is, he knows what the angel is holding. It may be John's part in the judgments (as joint heir with Christ - Romans 8:17 and Hebrews 9:15) to come. The judgment of a defeated Satan and fallen angels. The judgment of unbelievers in the seventh trumpet and the seven plagues (bowls).

And he placed his RIGHT FOOT on THE SEA and his left on THE LAND;

The right hand and foot is the side of authority. Thus placing the right foot on the sea indicates that this angel is in control of all spirits residing in the water. See brief discussion in Revelation 9:14. His foot on the land indicates this angel is in control of all the earth.

The promise in Genesis 17:1-8 given to Israel said that the land wherever they sojourned (walked) would be theirs. This angel as messenger to the whole world indicates by his stance that Christ and his redeemed will claim the whole earth in upcoming events. Cross references: Psalm 72:7-11, Psalm 110:1-2, and Hebrews 1:10-12. And Christ has the power to do it, Psalm 95:1-5.

Hallelujah!