Looking for a title I can not recall

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But I do remember the story to an extent.
>its like the 1920s
>two twin or very near age sister & brother
>dad is some sort of sleuth or conspiratorial agent
>dad goes missing I think?
>kids look for him and find cool maps n' shit
>find him? I think?
>also found the treasure everyone was looking for too
These books also came with some really badass illustrations and even diagrams of rotating/hidden book shelves and ciphers. If memory also serves me right there were three books in this series. I read these back in 8th grade so this is all I can remember. Hope thats enough.