Alternamorphs is a spinoff two-book series from the main 54 book Animorphs series. The books consist of a Choose Your Own Adventure "game book" style, where the character You are a sixth Animorph, who must choose how to react in certain situations, which will result in you living, dying, or getting stuck in morph. 

The following books are part of the Alternamorphs series:

1. The First Journey
2. The Next Passage

Gallery

Ad for book 1 The First Journey

Scholastic.com/animorphs web site summary, using the header image on the cover of both books, for book 1.

Ad for book 2 The Next Passage

French cover

German cover for a combined edition of books 1 and 2

Scholastic book order cutout for The First Journey