Re AS it is definitely worth reading because it was written so close after events, it relies on material from the investigation, and he is a respected journalist. When you read it you will understand why it upset Diana L. Her influence over the investigation does come through in his book and it's both sad--she was a scared and grieving mum--and shocking--she became a media personality, tried to influence the police over her daughter's lifestyle, was getting assistance from mediums and at one stage believed in a very silly conspiracy.
Re DV, your assessment of his podcast is sadly mine for the most part about his book. HIs book is more so. He presents the information in such a way as to direct the reader towards a particular conclusion, even when in doing so he is a bit economical with the truth.
For example, at one point he interviews one of the CID policemen who were on the original investigation and asks him if he was sent to Shorrolds Road, the officer says he wasn't. DV makes a huge moment of this, as if the police were so stupid they didn't even search Shorrolds Road, with the unspoken implication being that they didn't think Suzy had gone there. In reality, this is not true and according to AS uniformed officers were sent there straight away, searched inside and out (so gained access) and an officer stationed outside to guard it that evening.
His presentation of the temp relief landlord as shifty and his ex partner as shifty and suspicious is not fair at all, and clearly designed to push towards a conclusion.
If you read his interview with Adam L, it is fairly clear why he walks out. Adam says he believes the police that Cannan was involved. DV presses him on issues around how the police treated him, that he was considered a suspect at first, what happened with his last movements with Suzy on the Friday, did he go to the pub with her. If I were Adam I would read this as "this guy thinks I might be involved and is trying to get me to say something to show that". I would certainly be annoyed and not see any point in staying. He should have been honest about why he was asking the questions and asked Adam if he was OK to discuss these issues, it's not a witness interview, it's a chat with a member of the public (DV).