Just a quick note: for those who might have been interested in my textbook, Mathematical Methods for Optical Physics and Engineering, but who found the list price a bit expensive, Cambridge University Press is selling it for 30% off until April 30th!
If my calculations are correct, £38.50 = $62, so it’s quite inexpensive right now — grab a copy while you can!
I should also note that Cambridge has put a large number of books in their optics catalog at 30% off until the end of the month. Some great titles looks to be available for really cheap (including mine).
Skulls in the Stars 
Wow, it’s still expensive for me despite 30% off! Haha!
By the way, deer prof. The extra set of problems is available now? To be honest, I felt a little hard to continue reading your book without practice, because I can’t keep in mind some formulas, like those in Chapter 3.
Hi — haven’t had time to work on extra problems yet — it’s been a crazy semester! Still hoping to do some more book-related work over the summer, which will start next week.
Anyway, thanks again, professor! And expecting your new excellent book, Haha!