I've forgotten the name, but a certain author wrote this novel.
Literal
Name [topic-は] forgot [completion-しまい] [but-が], a certain author [subject-が] this novel [object-を] wrote [explanatory-のです].
ある + 作家 = 'a certain author,' a way of referring to a specific but unnamed person. は on 名前 topicalizes it as a contrastive element: 'as for the name, [I forgot], but the novel exists.' のです closes with explanatory emphasis.