Is Gordon Thomas a reputible source?

I have recently began reading Gordon Thomas's "Gideon's Spys" in the book he discusses various conspiracies, mainly addressing the Mossad but there seem to be plenty of other "dirty hands". He includes interviews with self proclaimed ex Agents of various agencies and well as with Mohamed Al-Fayed, which personally I don't think Mr. Al Fayed would grant interviews to any random person, but who knows. Does anyone know of this author? Is he a realiable source? As I said I have only just began reading the book and I don't know how, um, seriously I should take it or if it will require a lot of leg work to find the truth within it.

Thank you