im confused xd

then the next move is a book move??

i thought that after inaccuracy/mistake then the book moves will stop

yu-yu-yu-yu-yu-you iz dum

2 Nc3? is a missed win: 2 Nxe5 instead wins a central pawn.
After 2 Nc3? Nc6 the position transposes to the same position reachable after e.g. 1 Nf3 Nc6 2 Nc3 e5

makes sense

but how did black get a book move after

If 1 Nf3 Nc6 2 Nc3 e5 is in the book, then 1 Nf3 e5? 2 Nc3? Nc6 transposes to it, so you have more book moves after that.
Likewise 1 Nf3 e5? 2 e4? transposes to book positions.

My guess is you reached the same point in an opening which would normally follow book moves, but you reached it in the wrong move order giving him a book move in that position but you an inaccuracy because the move order was switched to reach a familiar position.

alr thx that solves it