A method for sewing signatures into book blocks in a bookbinding sewing machine, wherein for each of the book blocks the method comprises: forming one or more stitchings (230,235) between the signatures of the book block with corresponding sewing threads by means of one or more needles and one or more crochets, wherein the stitchings (230,235) comprise one or more rows of plain stitches formed by the needles and one or more rows of looped stitches formed by the crochets being concatenated to each other, the rows of looped stitches ending with corresponding free loops on a last one of the signatures (115 N) of the book block (155 f), bringing the sewing threads, upstream of the corresponding plain stitches along corresponding feeding directions of the sewing threads from corresponding spools, each to a catching position aligned with a corresponding one of the free loops formed by the sewing thread along an advancement line of the signatures during the sewing thereof, bringing the sewing threads each from the catching position to a concatenation position by passing the sewing thread through the corresponding free loop along the advancement line, characterized by bringing the sewing threads by means of corresponding one or more hooks each from the concatenation position to a closure position away from the corresponding free loop thereby tightening the free loop around the sewing thread being passed therethrough, the sewing thread being free to slide relative to the corresponding hook during said bringing the sewing thread from the concatenation position to the closure position, and cutting the sewing threads each upstream of a portion thereof being tightened by the corresponding free loop along the feeding direction, the sewing threads being cut after said bringing the sewing threads each from the concatenation position to the closure position. forming one or more stitchings (230,235) between the signatures of the book block with corresponding sewing threads by means of one or more needles and one or more crochets, wherein the stitchings (230,235) comprise one or more rows of plain stitches formed by the needles and one or more rows of looped stitches formed by the crochets being concatenated to each other, the rows of looped stitches ending with corresponding free loops on a last one of the signatures (115 N) of the book block (155 f), bringing the sewing threads, upstream of the corresponding plain stitches along corresponding feeding directions of the sewing threads from corresponding spools, each to a catching position aligned with a corresponding one of the free loops formed by the sewing thread along an advancement line of the signatures during the sewing thereof, bringing the sewing threads each from the catching position to a concatenation position by passing the sewing thread through the corresponding free loop along the advancement line, characterized by bringing the sewing threads by means of corresponding one or more hooks each from the concatenation position to a closure position away from the corresponding free loop thereby tightening the free loop around the sewing thread being passed therethrough, the sewing thread being free to slide relative to the corresponding hook during said bringing the sewing thread from the concatenation position to the closure position, and cutting the sewing threads each upstream of a portion thereof being tightened by the corresponding free loop along the feeding direction, the sewing threads being cut after said bringing the sewing threads each from the concatenation position to the closure position.