In this book:

A young physics student named April finds her life thrown into chaos when she barely survives an air disaster and meets Mark, a time traveller whose responsibility it is to protect the universe from those who would change history. They must find allies and use every resource to root out an infiltration of American society and save the Earth from being the victim of a future war.

Apprentice is currently available on Kindle and in paperback:

Kindle:
US: https://www.amazon.com/dp/B01MYZ1VA5
UK: https://www.amazon.co.uk/dp/B01MYZ1VA5
DE: https://www.amazon.de/dp/B01MYZ1VA5
FR: https://www.amazon.fr/dp/B01MYZ1VA5
ES: https://www.amazon.es/dp/B01MYZ1VA5
IT: https://www.amazon.it/dp/B01MYZ1VA5
NL: https://www.amazon.nl/dp/B01MYZ1VA5
JP: https://www.amazon.co.jp/dp/B01MYZ1VA5
BR: https://www.amazon.com.br/dp/B01MYZ1VA5
CA: https://www.amazon.ca/dp/B01MYZ1VA5
MX: https://www.amazon.com.mx/dp/B01MYZ1VA5
AU: https://www.amazon.com.au/dp/B01MYZ1VA5
IN: https://www.amazon.in/dp/B01MYZ1VA5

Paperback:
US: https://www.amazon.com/dp/1520142986
UK: https://www.amazon.co.uk/dp/1520142986
DE: https://www.amazon.de/dp/1520142986
FR: https://www.amazon.fr/dp/1520142986
ES: https://www.amazon.es/dp/1520142986
IT: https://www.amazon.it/dp/1520142986
JP: https://www.amazon.co.jp/dp/1520142986