SPI.transfer() reads and writes at the same time. Your device receives and transmits 16 bits. See page 30 of your manual.
Here is an example of sending and receiving 16 bits (2 bytes). The bytes you are sending are sendHighData (0xff) and sendLowData (0x00), and the bytes you receive from the device will end up in highData and lowData.
byte sendHighData = 0xff;
byte sendLowData = 0;
byte highData = SPI.transfer(sendHighData);
byte lowData = SPI.transfer(sendLowData);