The expression 'mentally diseased is not taken from the bible (any bible). It is from the July 15th 2011 Watchtower.
There it is being applied to people (they call apostates) who leave the organisation and actively speak against the governing body, to the extent of gaining followers for themselves.
However in the past the society view as apostates, anyone who just speaks out against the org. It has not mattered if they had followers or not. In fact in 1980 they said in an elders letter that just believing something different doctrinally could make someone an apostate. See below
September 1, 1980
TO ALL CIRCUIT AND DISTRICT OVERSEERS
Dear Brothers:
Keep in mind that to be disfellowshiped, an apostate does not have to be a promoter of apostate views. As mentioned in paragraph two, page 17 of the August 1, 1980, Watchtower, "The word 'apostasy' comes from a Greek term that means 'a standing away from,' 'a falling away, defection,' 'rebellion, abandonment. Therefore, if a baptized Christian abandons the teachings of Jehovah, as presented by the faithful and discreet slave, and persists in believing other doctrine despite Scriptural reproof, then he is apostatizing. Extended, kindly efforts should be put forth to readjust his thinking. However, if, after such extended efforts have been put forth to readjust his thinking, he continues to believe the apostate ideas and rejects what he has been provided through the 'slave class, then appropriate judicial action should be taken.
I too got rid of most of my books when I left. However most are available on line now. Just google the one you want.