Alex Rider Author Anthony Horowitz wrote Fledgling Jason Steed?

My pays mom says Anthony Horowitz wrote Fledgling Jason Steed and the name on the book Mark Cooper is just a pen name. She says he is writing the Jason Steed series now that he has finished Alex Rider.
1. Is this true?
2. Why do they write with a pen name?
3. When is the next Jason Steed book out?
Update: it should say my pals mom.
Update 2: Anyone? I thought it was a simple question, where would I find out?
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