The Angel who stood on the sea and on the land had a little book in his left hand (Revelation 10:2, cf. verse-5; Ezekiel 2:9). Although many scholars don’t believe the book in the Angel’s hand is the same book that was sealed in Revelation 5, it seems to me that they are the same. I think this is why John specifically pointed out that this little book was opened (Revelation 10:2). At this point in the Apocalypse the sealed book of chapter five would have been open with all of its seals broken.
John describes the book in the hand of the Angel as a little book (cf. Revelation 5:1; 10:2, 8-10). In the Greek it is biblaridion (G974), and it occurs only here in Revelation 10:2 and in verses 8-10. Most commentaries differentiate it from the book, biblion (G975), in Revelation 5:1, for the reason that it is little in chapter 10, but not so described in chapter 5. However, since biblion (G975) is also used for a bill of divorcement (Matthew19:7; Mark 10:4), there really isn’t an adequate cause to make biblion (G975) a “large book” in contrast with biblaridion (G974) a “little book.” Both references (Revelation 5:1 and 10:2) probably point to the same book.
The Angel “set his right foot upon the sea, and his left foot on the earth” (Revelation 10:1-2), which indicates his supreme authority, having all things are under his feet. His position indicates he is facing west with the Euphrates behind him (cf. Revelation 9:14-16), and judgment (upon Jerusalem) before him. So, in Revelation 10:6, where he swore by him who created the earth and the sea and all that in them are, he stood upon that which he created with his own hands (cf. John 1:3; Colossians 1:16-17). This picture is similar to the LORD on earth calling down judgment upon Sodom and Gomorrah from the LORD out of heaven (Genesis 19:24), but in the context of the Apocalypse that judgment would be upon Jerusalem (Deuteronomy 32:1-2; Psalm 65:5).
“A book, as such, thus borne in the hand of an angel coming down to the world, would be an indication that something of importance was to be communicated to people, or that something was to be accomplished by the agency of a book. It was not, as in Revelation 6:2, a bow – emblem of conquest; or Revelation 6:4, a sword – emblem of battle; or Revelation 6:5, a pair of scales – emblem of the exactness with which things were to be determined; but it was a book… The natural interpretation, then, would be, that something was to be accomplished by the agency of a book, or that a book was to be the prominent characteristic of the times – as the bow, the sword, and the balances had been of the previous periods.” [BARNS NOTES]
In other words, a new modus operandi may be in view in heaven’s relationship with mankind. So, as the Angel stood facing Jerusalem with the little book in his hand, he spoke with a loud voice, as though from a lion (Revelation 10:3; Isaiah 42:13; Jeremiah 25:30; Hosea 11:10). What the Angel said is not recorded, but the symbolism used in referring to the lion seems to indicate the state of awe and attentiveness the world would give that voice, just as what might be given to the roar of a lion that one hears in a forest or jungle.
In reply to the Angel’s loud voice, “…the Seven Thunders uttered their voices.” cf. Revelation 8:5; John 12:28-29). It is interesting that Psalm 29:1-11 records the seven voices of the Lord, where it is said he thunders like the sound of many waters, breaks the cedars of Lebanon and shakes the wilderness etc. However, just as the seven churches (Revelation 1:4, 11, 20) are really ONE church, and the seven spirits of God (Revelation 4:5) are really ONE Spirit, so the Seven Voices (thunders) of God are really ONE Voice (thunder). The number seven is used to denote completeness.
I think that such emphasis placed here upon the mighty voice of the Angel and its echo thundering out of heaven is meant to establish the importance of the little book in the hand of the Angel. Opening it has produced many judgments upon the persecutors of the saints, as well as to provide safety for the elect, as those judgments were carried out. Moreover, it was the message of the saints, which is also contained in that little book that warned their persecutors of their impending judgment, if the message went unheeded. Judgment occurred and would now be brought to its swift conclusion (Revelation 10:5-6), because the persecutors of the elect were, indeed, unrepentant (cf. Revelation 9:20-21).