Why are moths brown?

Not all moths are brown, many are very colorful.

I would say that it's a natural evolution so they blend in with the bark of trees when they land. That way it's harder for birds and other natural predator to see them. Many animals have developed to look like their habitat to protect them from being eaten.

Oh! I didn't know that. Hey, I have another question: Could I find textbooks on entomology in a library?

I'm sure they will have them. Depending on the library you go to. If you have access to a large library it will be better. Usually a library at a university is very diverse.

You should be able to find them at a public library... if they don't have them on hand, most libraries participate in some kind of book exchange program and may be able to get them for you, or point you to another library nearby that does have it. Ask your librarian.

Another option is a college library... many state schools have their libraries open to the public (although usually they have limited circulation and allow only partial use of the facilities to non-students).

A third option is, if you're looking for something to keep, hop onto Amazon and get an older edition of a textbook... many older editions of textbooks can be had for little more than the cost of shipping, and since you don't need the current end-of-chapter exercises, they'll be more than adequate.