Ronald Reng's book A Life Too Short, chronicling Enke's suicide in 2009 was voted William Hill Sports Book of the Year in London on Monday.
Speed was found hanged at his home on Sunday at the age of 42.
Reng said Enke's depression-driven suicide had changed attitudes about mental illness in Germany in the macho world of professional football.
"The first team Robert played for was Monchengladbach in the mid 1990s and through writing the the book I found out that in that one team there were five players who suffered from depression or mental illnesses and not one of them was able to address it publicly at the time," Reng told Reuters.
"These days in Germany we have weekly, sadly, people from the football world opening up and saying it.
"Former player Reinhold Mathy told me that in the old days it was referred to as a "hamstring" and it still happens that people have to hide it because football is all about strength, both physical and mental, and weaknesses have to be hidden."
Since the death of Enke, Reng said depression was no longer such a taboo subject in football.
"Before in Germany the only high profile case was of Sebastian Deissler who was the big promise of German football but he dropped out of football after admitting clinical depression," Reng said.
"After Robert's death the network of sports psychologists is much better. There are helplines, there is much higher awareness."
Reng said his close friendship with Enke opened his eyes to the "old school" attitudes about mental illness and he suggested that high-profile footballers were often in more danger from the damaging, potentially life-threatening, symptoms of depression.
HIDE INNINGS FEELINGS
"People ask me are professional footballers more prone to depression? I think not. It can hit anybody but what I will say is that professional footballers are trained all their life to hide their inner feelings, their real feelings, and that makes it so much more dangerous for them because they are bound to hide their disease better than anyone else.
"Robert wanted to write the book with me because it was a relief for him that one day he could talk about his depression because it was certainly a strain on him that he couldn't talk about it. He often wrote in his diaries how he wanted to shout it out to the world, but he had to lock himself in because Germany's number one [goalkeeper] is not expected to be depressed... he was supposed to be the last line of defence."
Speed won 85 caps for Wales and played with distinction for Leeds United, Everton, Newcastle United and Bolton Wanderers.
"I'm not comfortable to talk about Gary Speed because I don't know much about him other than remembering him playing for Newcastle alongside Didi Hamman and playing very well," the Barcelona-based Reng said after collecting his award.
"It would be disrespectful to his family for me to talk about it but as I said we shouldn't generalise and we have to wait and give the family the time to talk about t
