While the affixing of the mezuzah is a mitzvah set forth in the Torah itself, there are other things, too, that give a house a "Jewish appearance."
One of the most important is sifrei kodesh (Jewish holy books).
Some Jewish families for various reasons have actually kept sifrei Torah (Torah scrolls) in their homes.
Today, books other than the Scriptures are significant as well.
The presence of books in the house, and their accessibility, are not just a matter of convenience for reading and study.
Their very conspicuousness is a reminder that they should be used—at the very least looked into from time to time.
Children are tempted to open and delve into them.
Books generate a special atmosphere.
–Rabbi Adin Steinsaltz
From "The Home" in Teshuvah: A Guide for the Newly Observant Jew by Rabbi Adin Steinsaltz