There are several books mentioned in the Bible which are not part of the Bible that we have today.
Book of the wars of the LORD (Numbers 21:14)
Book of Jasher (Joshua 10:13 & 2 Samuel 1:18)
Book of the acts of Solomon (1 Kings. 11:41)
Book of Samuel the seer (1 Chronicles 29:29)
Book of Nathan the prophet (1 Chronicles 29:29 & 2 Chronicles 9:29)
Book of Gad the seer (1 Chronicles 29:29);
Prophecy of Ahijah (2 Chronicles 9:29);
Book/Visions of Iddo the seer (2 Chronicles 9:29; 12:15; 13:22)
Book of Shemaiah (2 Chronicles 12:15)
Book of Jehu (2 Chronicles 20:34)
Sayings of the seers (2 Chronicles 33:19)
A letter to Laodicea (Colossians 4:16 - although some scholars feel this letter is the one currently entitled Ephesians.)
A prophecy of Enoch, known to Jude (Jude 1:14)
This doesn’t mean that the Bible is incomplete. The scriptures which compose the Bible were written by inspiration of God and we can be confident that God ensured that the scriptures necessary for our salvation are all present and correct.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the man of God may be perfect, throughly furnished unto all good works.
The gospel is complete
1 Corinthians 15:1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
Galatians 1:8-9 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. 9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
The doctrine is complete
Acts 2:42 And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.
1 Timothy 4:16 Take heed unto thyself, and unto the doctrine; continue in them: for in doing this thou shalt both save thyself, and them that hear thee.
2 John 1:10 If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed:
There are other books known as apocrypha which have never been accepted as inspired by God.
Go here for details: https://en.wikipedia.org/wiki/Biblical_apocrypha Reading them is sufficient to highlight the difference from the inspired writings.
We need not fear that some vital piece of information is hidden in a book that we do not possess as part of the currently accepted books of the Bible.
I hope you find this helpful.