Read through Johnny's conversation with Nick and Isabelle. You'll receive a memory link after the conversation.
Head downa to the bookshelves and Johnny will start walking towards River, there's going to be a brief chatter between them afterwards.
After River goes to the cashier, interact with Johnny. He'll then split up into three versions of himself around the bookshelves.
Firs talk to the Johnny that's on the same aisle as River before she left, then go to the south where the other 2 Johnnys are.
Speak to the Johnny on the right side first. That little girl that he's talking you will ask you about what Animorph David is. You don't know the answer so just key in 1 letter, press okay, then leave.
Now speak to the Johnny on the left side and he'll move over beside River on the counter.
Before you interact with them, check out the bookshelf where the invisible/unimportant person is looking. The bookshelf will give you a memory link.
Talk to Johnny by the counter and he'll ask River about the Animorphs. Note down David's morph and speak to the girl again, then key in the animal to get the last memory link.
Go back to the counter once you have all the memory links and use it on the book that River is purchasing. It's on the right hand side of the counter.
Press the yellow and orange button on the left side of the puzzle, then press the diagonal button. Finish up by pressing the green and yellow button at the bottom of the puzzle.