Even if the book could be considered canon, it's not showing me that Spongebob likes her.
So the summary goes like this: "SpongeBob Squarepants really wants to get a special valentine for his friend Sandy. However, Spongebob's other friend, Patrick Star, informs him that if he gives Sandy a special gift it will change his relationship not only with Sandy, but with all his other friends as well. Could Patrick be right?"
Okay, so a few things.
1. Patrick is the one who says that it'd cause subtext, not Spongebob. Sponge may just want to give a gift to his friend and Patrick si the one who thinks it's something else, and we all know he lacks in the brain department. Heck, the book may be about how people read into things too much when a boy and a girl are friends and assume things. All kid's books like this have some sort of moral.
2. Spongebob simply giving Sandy a valentine doesn't prove he likes her. He gave one to Patrick in an episode after all.
I guess I'd have to actually read the book to know, but just the summary doesn't prove Spongebob even liking Sandy in the book, much less canon.
Believe me, I wish he did. I'm a shipper of those two, but the book isn't proof.