The Hermes soap that is used to clean saddles is very good in cleaning leather.
The official name is "Pain de Savon Glycerine" or "Savon H a la Glycerine" (Sodium Soap in English). It says that it is made out of fatty acids of animal. It's about 14cm long and rectangular/square in shape. It's like a transparent beige/yellow similar to the Neutrogena soap.
It's great. You just need to slightly wet a towel, rub a bit of the soap, and gently apply in the leather.
I used the soap mostly for my other bags as my only Hermes bag, the Kelly, is squeaky clean!