The texts that make up the Nag Hammadi library are not books per se but a group of scrolls collected into four codices, or bound books. More than 50 texts comprise these codices. I won't list them all for you; you can find all the titles at the link in the sources section.
The most famous scrolls in these collections are what are called the Gnostic Gospels, which include the Gospel of Thomas, the Gospel of Philip, and the Apochryphon of John. The word "gospel" simply means "good news" and does not necessarily signify a holy text; thus their exclusion from the New Testament. The term "gnostic" means "knowledge" or "learning," suggesting that these texts contain hidden wisdom.