Yamaguchi was diagnosed in February, unfortunately the cancer was advancing rapidly and surgery wasn't a feasible option. Instead he was put on a course of anti-cancer medication.
Fortunately it would appear the medication has been quite effective, during surgery to remove gallstones (which Yamaguchi had later been diagnosed with around May, unrelated to the cancer) it was confirmed that the cancer had shrunk. The anti-cancer treatment was continued, and recently it was determined that surgery to remove the cancer is now possible. An operation has been scheduled for early August.
The author has apologized to his fans regarding volume 21 of Zero no Tsukaima, explaining that because of his illness the August release of the volume has become difficult to sustain. He has stated however that the structure for volumes 21 and 22 (the final volumes in the series) has already been planned out and he will finish them both as soon as he's back to full health.
His fans have been sending their best wishes to Yamaguchi's twitter and by post to an address provided with the announcement. Hopefully the operation will be a complete success and the creator of tsundere princess Louise will be back to full strength in no time.
For those interested, the original Japanese announcement can be found here.