The cardigan is worked top down, with set in sleeves worked simultaneously as described by Barbara Walker. The method is closely related to raglan sleeves but in stead of increasing on both bodice and sleeves, increases are only made on the sleeve side.
The resulting sleeve shape is a trapezoid, which is close enough to the shape of an ideal set in sleeve to ensure a good fit. And the finishing is beautiful all in all very nice, in my opinion. I have done a review of different methods for knitting set in sleeves, including the contiguous method by Susie Myers, will share my thoughts when I get around to it.