Doctor Zero, I strongly recommend the recent "Back Issue #17", published by Two Morrows Press. (See
www.twomorrows.com ) There's an extensive article covering Kara Zor-El's career from "Adventure Comics" to "Crisis on Infinite Earths #7", with insider details on why DC kept changing Supergirl... and finally killed her off. :cry:
There's also an examination of the still-born "Double Comics", which was to combine the suddenly-cancelled Superboy and Supergirl books until DC decided that neither character would exist after Crisis and the Byrne reboot. :evil:
According to the Supergirl article, when Mort Weisinger retired in 1970, his tightly-held control of all the Super-titles were divvied up among the DC staff. Mike Sekowsky was apparently chosen because his work in "Metal Men" and "Wonder Woman" had made these characters more "human", and it was hoped he would also work similar magic on Supergirl.
Granted, I understand you dislike Sekowsky's handling of the Metal Men, and the depowered mod "Diana Prince" era was completely alien to the "traditional" Wonder Woman mold, but remember this was a common theme in the Seventies -- more "relevant" issues (racial and women equality, the war on poverty and drugs, pollution, etc.), less costumed supervillians and more "ordinary" criminals or aliens, focusing more on a hero's private life and personal problems, and so on. So rendering Supergirl's powers "on-again, off-again" was in line with Wonder Woman turning into DC's "Emma Peel", and Superman (temporarilly) getting his powers halved. However in Supergirl's case, the execution was clumsy and the superpowered armature Kara had to wear under her costume in case her powers conked out again (ALWAYS at an inconvenient time) was even more ill-conceived.
Then again, Sekowsky also created Nastalthia "Nasty" Luthor, who despite her continuity-jarring origin (Lex had ANOTHER sister?) has become one of Kara's most-remembered antagonists. And Nasty has just been rebooted in "All-Star Superman #5". So I'll give Sekowsky some credit.