Invisible ink is any chemical that disappears when a message is written in it and that can be made to reappear later by applying heat or some other chemical. The invisible ink is usually put inside a fountain pen in place of regular ink.
A common invisible ink is lemon juice, which leaves no visible trace when it is written with but can be revealed by gently heating the paper. Estelle’s use of whisky for the same purpose is unconventional, but we can perhaps assume that whisky is more readily available than lemon juice in Dunsayn Manor.
More subtle invisible inks include vinegar and ammonia, both of which become visible when a light solution of red cabbage water is applied. However, ammonia must be applied carefully or it can bleach the paper - a sure giveaway.
Because a book of blank pages can look a bit suspicious, people who write in invisible ink often put their secret message between the lines of a “decoy” text written in ordinary ink. Estelle’s secret diary, for example, is written between the lines of what appears to be a boring account of what she ate and wore each day.