The Case of the Missing Marquess by Nancy Springer

More slowly I wrote another note, one that would soon wing for miles via wire to be printed out by a teletype machine as:

LADY EUDORIA VERNET HOLMES MISSING SINCE YESTERDAY STOP PLEASE ADVISE STOP ENOLA HOLMES

I directed this wire to Mycroft Holmes, of Pall Mall, in London.

And also, the same message, to Sherlock Holmes, of Baker Street, also in London.

My brothers.

On Enola's fourteenth birthday, her mother leaves without a word or a note, only three gifts: a fully-supplied drawing kit; a book on the meanings of flowers and other things; and a small book of ciphers.

Ciphers are word puzzles; for example, Enola's name is a very simple cipher. When spelled backward, it spells 'alone,' as Enola has often found herself, quite content. Or, for another example, ALO NEK OOL NIY MSM UME HTN ASY RHC is the first cipher Enola solves to find the message: Enola, look in my chrysanthemums.

Armed with the information left in these gifts, Enola sets out to find her mother, ignoring the plans Mycroft has set out for her (a boarding school? Not on her life) and hopefully outsmarting the great detective Sherlock Holmes.