The Secret at the Gatehouse by Carolyn Keene

Louise and Jean become enmeshed in another mystery when they chance to meet Mrs. Zerbe, who used to work for Mr. Warrington, one of the owners of Captain Dana's ship. Mrs. Zerbe is devastated that she lost her job at Mr. Warrington's estate. During her conversation with the Danas, Mrs. Zerbe mentions a secret which she has kept for Mr. Warrington.