Is the scroll/book revealed in Revelation 5:8-9 the same scroll/book that was sealed in Daniel 12:4, 9?
Short Answer: Yes. It seems most plausible that the book being opened beginning in Revelation 5 is the very same that was sealed in Daniel 12. Here is the first mention of this book:
Revelation 5:8-9: "When He had taken the book, the four living creatures and the twenty-four elders fell down before the Lamb, each one holding a harp and golden bowls full of incense, which are the prayers of the saints. 9And they sang a new song, saying, 'Worthy are You to take the book and to break its seals; for You were slain, and purchased for God with Your blood men from every tribe and tongue and people and nation.'"
In the Book of Revelation (chapter 10), a mighty angel has taken his stand on the sea and on the land. The angel raises his right hand to heaven and swore that there would no longer be delay. What would no longer be delayed? The mystery of God announced by the prophets would be fulfilled. When would this happen? The angel says in the days when the seventh angel blows his trumpet. Remember that we have been listening to the seven trumpets blasting judgment. We have read of six trumpets sounding up to this point. The angel then declares that when the seventh angel sounds his trumpet then it is done. The mystery of God spoken by the prophets will be fulfilled.
To gain further clarity about what we are being told, we must recognize that this picture is found in Daniel 12, were we see the same angel taking the same stand and making the same oath:
Daniel 12:4, 6-7, 9, NASB: "But as for you, Daniel, conceal these words and seal up the book until the end of time; many will go back and forth, and knowledge will increase... 6And one said to the man dressed in linen, who was above the waters of the river, 'How long will it be until the end of these wonders?'
7I heard the man dressed in linen, who was above the waters of the river, as he raised his right hand and his left toward heaven, and swore by Him who lives forever that it would be for a time, times, and half a time; and as soon as they finish shattering the power of the holy people, all these events will be completed. 8As for me, I heard but could not understand; so I said, 'My lord, what will be the outcome of these events?' 9He said, 'Go your way, Daniel, for these words are concealed and sealed up until the end time.'"
Now, let us consider Revelation 10 in this same vein:
Revelation 10:5-7, NASB: "Then the angel whom I saw standing on the sea and on the land lifted up his right hand to heaven, 6and swore by Him who lives forever and ever, WHO CREATED HEAVEN AND THE THINGS IN IT, AND THE EARTH AND THE THINGS IN IT, AND THE SEA AND THE THINGS IN IT, that there will be delay no longer, 7but in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished, as He preached to His servants the prophets."
All the elements of Daniel 12:7 and Revelation 10:5-7 appear to parallel one another. The only significant difference is that of the timeline. When the angel in Daniel proclaims his oath, he declares that it was going to be a “time, times, and half a time.” A period of time was going to pass before these events would unfold. However, the real point that is repeated in Daniel 12 verses 4 and 9 is that the words were to be concealed and sealed up until "the end time." That is, the prophetic fulfillment would not occur in the days of Daniel, but rather later during the "end time." What is the "end time" if not the Church Age?
The angel reveals in Daniel what event his oath concerns. Observe the end of verse 7, “[As] soon as they finish shattering the power of the holy people, all these events will be completed.” When the angel said these words to Daniel, who were the “holy people?” "Holy people" was a reference to the nation of Israel. As we look to the Book of Daniel, we see the same point being made:
Daniel 12:1b: “And there will be a time of distress such as never occurred since there was a nation until that time; and at that time your people, everyone who is found written in the book, will be rescued."
The angel tells Daniel that this prophecy is about his people, the Jewish nation. Those who were written in the book: Christians, would be delivered from this time of great tribulation. As we again look to the Book of Revelation, the angel declares:
Revelation 10:6c-7: "[There] will be delay no longer, 7but in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished, as He preached to His servants the prophets."
Which specific prophet might the angel have in mind? The most obvious answer is Daniel. The angel further told Daniel that there would be delay ("a time, times, and half of time," Daniel 12:7). Now an angel tells John there will not be a delay (Rev. 10:6).
This angel also seems to confirm that the locusts in Revelation 9 represent a world power destroying another nation in Scripture (cf. Joel 2). The logical interpretation of the locusts is that they represent the Roman Empire coming against the Jewish nation in A.D. 70. Revelation 10, in conjunction with Daniel 12 appears to validate this. Many clues reveal themselves as far back as chapter 6: the physical Jewish nation was the object of God’s wrath.
Jesus clearly prophesied of this doom in Matthew 24 (cf. Luke 21) even mentioning Daniel by name:
Matthew 24:15-21: "Therefore when you see the ABOMINATION OF DESOLATION [Roman Imperial Army] which was spoken of through Daniel the prophet, standing in the holy place (let the reader understand), 16then those who are in Judea must flee to the mountains. 17Whoever is on the housetop must not go down to get the things out that are in his house. 18Whoever is in the field must not turn back to get his cloak... 21For then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever will."
Paul also noted God's coming wrath against the nation of Israel:
1 Thessalonians 2:14-16: "For you, brethren, became imitators of the churches of God in Christ Jesus that are in Judea, for you also endured the same sufferings at the hands of your own countrymen, even as they did from the Jews, 15who both killed the Lord Jesus and the prophets, and drove us out. They are not pleasing to God, but hostile to all men, 16hindering us from speaking to the Gentiles so that they may be saved; with the result that they always fill up the measure of their sins. But wrath has come upon them to the utmost."
While some of the wording is slightly altered, it seems relatively clear that Daniel 12:4, 6-7, and Daniel 12:9 are referring to the same thing. When we tie many verses together from Daniel, Matthew, 1 Thessalonians, Revelation, and others, the message seems to indicate that that which was sealed in the book during Daniel's day is now being revealed in the Book of Revelation per verses 10:6c-7 and others. The picture it paints is that of the horrific judgment seen in the previous chapters against the nation of Israel in AD 70.