Revelation 10

Just as John recorded -

"...I saw in the right hand of him that sat on the throne a book written within and on the backside, sealed with seven seals." (New Testament | Revelation 5:1)

...suggesting that this book contained the events of the seven thousand years or in other words, the plan of salvation, John also records -

"...I saw another mighty angel come down from heaven, clothed with a cloud: and a rainbow was upon his head, and his face was as it were the sun, and his feet as pillars of fire:
2 And he had in his hand a little book open..." (New Testament | Revelation 10:1 - 2)

And the angel instructed John -

"...Go and take the little book which is open in the hand of the angel which standeth upon the sea and upon the earth.
9 And I went unto the angel, and said unto him, Give me the little book. And he said unto me, Take it, and eat it up; and it shall make thy belly bitter, but it shall be in thy mouth sweet as honey." (New Testament | Revelation 10:8 - 9)

Being the book with the seven seals was the plan of salvation of all men, I sensed this small book was a specific plan or calling particularly for John. I could almost compare this to a patriarchal blessing. At this time, John must have been in his nineties. A very aged man. We learn a little bit more of what must have been in the book as the Lord talks to three faithful disciples of the Nephites -

"...I know your thoughts, and ye have desired the thing which John, my beloved, who was with me in my ministry, before that I was lifted up by the Jews, desired of me.
7 Therefore, more blessed are ye, for ye shall never taste of death; but ye shall live to behold all the doings of the Father unto the children of men, even until all things shall be fulfilled according to the will of the Father, when I shall come in my glory with the powers of heaven.
8 And ye shall never endure the pains of death; but when I shall come in my glory ye shall be changed in the twinkling of an eye from mortality to immortality; and then shall ye be blessed in the kingdom of my Father.
9 And again, ye shall not have pain while ye shall dwell in the flesh, neither sorrow save it be for the sins of the world; and all this will I do because of the thing which ye have desired of me, for ye have desired that ye might bring the souls of men unto me, while the world shall stand." (Book of Mormon | 3 Nephi 28:6 - 9)

The bitterness in the belly that John would experience then would be the sorrow for the sins of the world. John thus recorded to himself -

"AND the Lord said unto me: John, my beloved, what desirest thou? For if you shall ask what you will, it shall be granted unto you.
2 And I said unto him: Lord, give unto me power over death, that I may live and bring souls unto thee.
3 And the Lord said unto me: Verily, verily, I say unto thee, because thou desirest this thou shalt tarry until I come in my glory, and shalt prophesy before nations, kindreds, tongues and people.
4 And for this cause the Lord said unto Peter: If I will that he tarry till I come, what is that to thee? For he desired of me that he might bring souls unto me, but thou desiredst that thou mightest speedily come unto me in my kingdom.
5 I say unto thee, Peter, this was a good desire; but my beloved has desired that he might do more, or a greater work yet among men than what he has before done.
6 Yea, he has undertaken a greater work; therefore I will make him as flaming fire and a ministering angel; he shall minister for those who shall be heirs of salvation who dwell on the earth.
7 And I will make thee to minister for him and for thy brother James; and unto you three I will give this power and the keys of this ministry until I come.
8 Verily I say unto you, ye shall both have according to your desires, for ye both joy in that which ye have desired." (Doctrine and Covenants | Section 7:1 - 8)

I'm sure this is what was written in the "little book".