There is more than meets the eye to everyone
One of the most significant themes in The Last Thing He Told Me is that there is more than meets the eye to everyone. In the novel, Hannah initially thinks her husband Owen is a caring man who is unwaveringly loyal to her and his daughter. However, she quickly learns that Owen has more going on than meets the eye: his name isn't actually Owen, he put incredibly dangerous people in prison, and he has disappeared at the drop of a hat - all things she didn't expect of him.
The Last Thing He Told Me's emphasis on people not being honest about who they truly are and being different than they seemed is included to ratchet up the tension and drama in the novel. This is also emphasized in the novel because to illustrate the importance of fully discovering who someone close to you really is.
The conflict between a step-parent and their step-children
An overarching theme in the novel involves the conflict between a stepparent and their stepchildren. In the novel, Hannah and Bailey initially butt heads frequently. Though Hannah tries to connect with her stepdaughter Bailey, Bailey is oftentimes resistant. She doesn't trust Hannah and isn't all that comfortable around her.
But the two begin to grow closer after the disappearance of Owen, Hannah's husband and Bailey's father. Their change from being distant to growing close together serves the story and shows audiences that being involved in a stepparent/stepchild relationship doesn't mean that the two people will always hate each other.
Sometimes, the government has your best interests at heart
A fairly common trope in film, television, and other popular culture involves the government acting in an evil way. However, a minor theme in the novel involves the government - through a U.S. Marshall - actually having the best interests of someone (in the case of the novel, Owen and his family) at heart.
