Fantasy:
* The Alchemyst (2003) - This was his debut novel and the first book in the The Alchemyst series.
* The Shaman (2005) - The second book in The Alchemyst series.
* The Sorcerer (2006) - The third book in The Alchemyst series.
* The Warlock (2008) - The fourth book in The Alchemyst series.
* The Magician (2011) - The fifth and final book in The Alchemyst series.
* The Keys to the Kingdom series (2007-2012) - A children's fantasy series consisting of five books.
Historical Fiction:
* The Song of the Exile (2016) - The first book in the The Song of the Exile trilogy.
* The Shadow of the Dragon (2017) - The second book in The Song of the Exile trilogy.
* The Fall of the King (2018) - The third book in The Song of the Exile trilogy.
Other:
* The Book of the Golden Serpent (2010) - A standalone fantasy novel for younger readers.
Nicholls has also written several short stories and contributed to various anthologies. He is a popular and critically acclaimed author, known for his engaging storytelling and well-developed worlds.