To expound on the other answer here is an in-depth-comparison of where each comes from. The first book of Napolean is <161 pages and maybe contains ~200 words per page. The Book of Mormon is 531 pages and contains ~450 words per page. (Both are rough estimates). Even if it was copied, that leaves several hundred pages that don't have a 'source'.
THE FIRST BOOK OF NAPOLEON
Condemn not the (writing)1...an account2...the First Book of Napoleon3... upon the face of the earth4...it came to pass5...the land6...their inheritances their gold and silver and7...the commandments of the Lord8...the foolish imaginations of their hearts9...small in stature10...Jerusalem11...because of the perverse wickedness of the people.12
BOOK OF MORMON
Condemn not the (writing)1...an account2...the First Book of Nephi3... upon the face of the earth4...it came to pass5...the land6...his inheritance and his gold and his silver and7...the commandments of the Lord8...the foolish imaginations of his heart9...large in stature10...Jerusalem11...because of the wickedness of the people.12
The First Book of Napoleon (taking quotes in order, some also appear earlier then mentioned)
1 page 9/161 condemn not the feebly imitative manner of writing therein occasionally employed
2 10/161 and an account of all thy deeds
3 11/161 THE FIRST BOOK OF NAPOLEON
4 13/161 The dominion of the Tyrant extendeth itself upon the face of the earth
5 19/161 And behold it came to pass
6 19/161 It seizeth upon the inhabitants of the land of Gaul.
7 20/161 their inheritances, their gold and silver, corn and oil
8 22/161 And they not only despised the commandments of the Lord
9 27/161 in the foolish imaginations of their hearts, had vainly worshipped
10 /161 And this man, though small in stature
11 31/161 and threatened the city of Jerusalem
12 29/161 because of the perverse wickedness of the people.
The Book of Mormon (also in order as much as possible)
1 title page condemn not the things of God
2 introduction The record gives an account of two great civilizations.
3 1 Nephi 1 THE FIRST BOOK OF NEPHI
4 1 Nephi 1:11 And they came down and went forth upon the face of the earth;
5 1 Nephi 1:12 it came to pass that as he read,
6 1 Nephi 2:4 And he left his house, and the land of his inheritance,
7 1 Nephi 2:4 and the land of his inheritance, and his gold, and his silver, and his precious things,
8 1 Nephi 2:10 firm and steadfast, and immovable in keeping the commandments of the Lord!
9 1 Nephi 2:11 And this they said he had done because of the foolish imaginations of his heart.
10 1 Nephi 4:31 And now I, Nephi, being a man large in stature,
11 1 Nephi 5:4 (10/531) but had tarried at Jerusalem, and had perished with my brethren.
12 Alma 59:11 (358/531) because of the wickedness of the people,