Which of the Regulars can tell me anything important?

Hint 1 of 4:

Inspector Lestrade invited you to come see him, so you should take him up
on that offer.

Hint 2 of 4:

You should also visit London Library for information on poisons.

Hint 3 of 4:

See H. R. Murray for information on the poisons used to kill Tuttle.