How does VSWR at a power amplifier output affect the level of harmonics? and is there any textbook or paper which addresses this issue?
If VSWR = 1, then voltage on the output amplifier is the "lowest" and RF matching is ok (RF amplifiers are designed delivering "always" maximum power on load). If VSWR gets higher, then voltage on output of amplifier may also be higher "some time" because of the reflected wave. And if it is higher, it can cause higher non-linearities (by clipping and/or breakdown)... Some solid RF amplifiers are protected through special circuitry which measure VSWR and protect them as soon as possible (quasi instantaneous).