I think that will be a 39 books. already we have 31 -
1st - 11th - the tomas part.
12th,16th-20th,24th-31th - the madrigal part
13th-15th, 21th-23th - the lucian part.
32th-35 - janus part (unstoppable) to be released...
36th-39 ? - the ekaterina part ? to be annouced...
and I think that each branch series is equal to a book series that no related to 39 clues.
lucian=kane chronicles (rick riordan)
tomas=percy jackson (rick riordan)
madrigal=heroes of olympus (rick riordan)
janus=the secrets of nicolas flamel the immortal (michael scott)
ekat= the george series (stephen hawking)