grammar - Can I use 'in' instead of 'at'? -- "I have more books in my house."

I am learning English using the Duolingo English course, one of correct solution was

I have more books at my house.

Can I use

I have more books in my house.

If it is OK, what is the difference between both?

Answer

It is a matter of "location" at my house vs. "contained/enclosed by/inside" in my house.
In your examples, house could refer to a location or what is inside it. Which one is best may depend on context.
For example, when you say:

I have more books at my house.

they could be on the driveway, in your backyard, etc. as these would still be referred to as your house location-wise.

When you say:

I have more books in my house.

they could be in a desk drawer, in your room, etc. stating what they are inside of.

You could say:

I have more books in my desk drawer.

but you would not say:

I have more books at my desk drawer.

But if you are specific about location:

I have more friends at the lake.

this probably does not mean the same thing as:

I have more friends in the lake.