Revelation 10- Those in the Book represent those who will inherit the Kingdom. It's NOT a huge book.

Rev 10:9
And I went to the angel, telling him to give me the little book. And he said^ to me, "Take it, and eat it; and it will make your stomach bitter, but in your mouth it will be sweet as honey."

Rev 10:10
And I took the little book out of the angel's hand and ate it, and it was in my mouth sweet as honey; and when I had eaten it, my stomach was made bitter.

This same command is given to Ezekiel. He is told to eat a scroll and it tasted sweet to his mouth.

Ezekiel 3:1-4 Then He said to me, “Son of man, eat what you find; eat this scroll, and go, speak to the house of Israel.” 2So I opened my mouth, and He fed me this scroll. 3And He said to me, “Son of man, feed your stomach, and fill your body with this scroll which I am giving you.” Then I ate it, and it was sweet as honey in my mouth. 4Then He said to me, “Son of man, go to the house of Israel and speak with My words to them.

Jeremiah tells us that accepting the Word of Elohim by faithfulness is "eating it"

Jer 15:16 Thy words were found and I ate them, And Thy words became for me a joy and the delight of my heart; For I have been called by Thy name, O LORD God of hosts.

The Book of Life given to John tastes sweet at first, but it made his stomach bitter. Those in the Book represent those who will inherit the Kingdom. Notice that it is a little book. It is NOT a huge book. It is mentioned three times in this passage that the book is "little." It is only for those who know Him, know His Son, and obey the words of the Father. The fact that it was small is likely what made it bitter to his stomach.