Mighty Jack and the Goblin King by Ben Hatke

Jack and his friend Lilly are searching for Maddy, Jack's autistic sister.  They had followed her through a portal and enter an eerie, mysterious world where goblins, rats and giants rule.  Will they all be able to return home safely?
My eleven year old and I both read this to ourselves independent of each other.
It is perfect for older, struggling readers as the text is limited and the visuals tell most of the story.
Points to discuss/warning: word "a**" is used as a swear word; Lilly kisses Jack on the lips.