May have written? Did she or didn’t she? If the author herself is not sure, then she probably did not write it.
Maybe yes, if she now has a disability that affects the memory, such as Alzheimers. Even in that case, there should be plenty of drafts around that led up to this ‘surprise’ book.
Unlikely there is a third book, but even more unlikely that there would have been drafts around. Lee was traumatized, not in a “I can’t go on living” way but still traumatized, by her sudden and near universal praise for her book. She stayed busy in the literary world, but could not bring herself to write again.
So even if she did write, it likely would have been a single draft that she kept out of view from anyone else. Like Emily Dickenson with her poems. Only after her passing was the volume and quality of her work revealed.