Apparently there is a third guide book coming for Pokémon Sun and Moon.
Pokémon Sun and Pokémon Moon: The Official Alola Region Collector's Edition Pokédex & Postgame Adventure Guide
Price: $33.76 USD
ISBN: 978-0744018141
Links to pre-order:
Amazon.de
Amazon.co.uk
Amazon.com
Bookdepository
Also on a site note, the Pokémon Sun and Pokémon Moon: Official Strategy Guide Collector's Vault is only $79.99 USD at
Amazon.com