Missing books referenced by who?
I do accept EVERY word from God’s “mouth” - and believe that it is all contained in the WORD of GOD known as the Bible.
You have taken your quote from John 21 completely out of context. What was being said by the author in this case? He was simply saying that not every act of Jesus Christ was written down (and considering I believe that the Bible is literally authored by God Himself, this makes even more sense) because the events were so plentiful. Indeed, if a detailed, hour-by-hour description had been kept of Jesus’ 3 year public ministry - that alone would be a huge amount of writing, not to mention all that He did and said when just speaking with the Apostles. Because God divinely guided the writing of His Book, He chose to only include that which was deemed necessary (and indeed, there are times in the Bible texts where the penman is instructed to NOT write certain things they have witnessed).
No amount of taking Bible text out of context will back up “adding to” the scriptures.
And consider this also (I bet you didn’t realize you were debating with a former LDS member, did you?) - I have, in years past, studied extensively the Book of Mormon - and one of the things that quite bothered me was some apparent conflicts that I just could not rectify between that book and the Bible. Some of the conflicts were simply variations in ideas, to downright disagreement.
Who are you to decide what the context of John 21 is? Peter tells us in 2 Peter 2:21 (exact chapter and verse not sure) that no scripture is of private interpretation--which is what you have done.
Since you now (apparently) embrace the beliefs of Protestant Christianity that denies current revelation, you cannot with any authority claim to interpret any scripture, because to do so (according to Peter) requires the spirit of revelation--which you deny exists.
No amount of rationalizing on your part can explain away your deficient understanding of the Bible, its completeness, or lack thereof. If you aren't aware of the many internal errors and contradictions in the Bible texts I won't waste my time explaining them to you. You need to do more study, both of the Bible and the Book of Mormon. There is no contradiction between the Bible and the Book of Mormon.