All? Artech House books are available on Print-on-Demand, so are never out of print, so far as I know.

https://us.artechhouse.com/Microwave-and-Millimeter-Wave-Diode-Frequency-Multipliers-P102.aspx

Interestingly, there are no/not many used copies around on the market at the moment:

https://www.bookfinder.com/search/?ac=sl&st=sl&ref=bf_s2_a1_t1_1&qi=mNlxUfTAfRwozx..Uago.VeFpCM_1497963026_1:2:2&bq=author%3Dmarek%2520t%2E%2520faber%253B%2520jerzy%2520chramiec%253B%2520miroslaw%2520e%2E%2520adamski%26title%3Dmicrowave%2520and%2520millimeter%2Dwave%2520diode%2520frequency%2520multipliers%2520artech%2520house%2520microwave%2520library