His books gave the most information at the time, he was closest to those investigating it at the time and he kept in contact with many of them for years later. His opinions at the end of his books on ALA aren't fully convincing but they're just opinions.
But either way Graysmith will be more in the know than you or I as to what contact Toschi and PH.
I'm no fan of PH don't get me wrong, he threatened to sue me - but I do have a soft spot for him for fully exposing Jackie to those with their eyes open for what she is. 
ALAs DNA does not match the Zodiac.
ALAs handwritting does not match the Zodiac.
ALAs palm prints don’t match the Zodiac.
ALA looks nothing like what the eyewitnesses describe.
ALA passed an intensive FBI polygraph examination.
All this except the DNA was known back then. Yet Graysmith thought he was onto something because ALA had a Zodiac wristwatch and the Letters stopped while he was in jail for a few month.
As for PH exposing Jackie. That’s new to me. What happened?