Answer:
1. The author hasn't told us the real reason of why Laura died. She has told us about their housemaid, Reenie, but we do not know how that is connected to Laura's death. Also, the author says that Laura might have her "reasons", but doesn't even tell us indirectly what those reason might be...
2. Two possible answers
- Laura's notebooks are her childhood diaries/a record of a secret relationship/a novel
OR
- Iris feels responsible for her sisters death/never loved her husband
Explanation for the 2nd one:
- The author remembers Reenie (maybe while reading one of Laura's diaries) and she also feels the need to tell Richard, her husband. But why does she feel the need to tell Richard? Maybe because Richard may have been in love with Laura, and because something went wrong (something didn't or couldn't work out) they have split up and Richard married Iris. Or maybe Iris felt some kind of anger towards Laura and that's why she married Richard (we don't quite know). And Laura may have been furious of Richard's or Iris' decision so much that she decided to kill herself (or did she? We don't know 'cause it's the first chapter). And that might be the reason Iris might have felt the need to inform Richard about Laura's death - because she thinks (or maybe knows feels) that Richard doesn't love her, he still loves Laura and that because they got married that, somehow may associate on the reason of Laura's death.
