Party of One: Leda (2003)

Party of One: Leda - Book the first of the Maduban Saga
is a paragraph-driven, solitaire adventure game, set in a fantasy world.

The adventure supposedly continues in Party of One: Dugal, but it seems that Party of One: Dugal has not yet been published.

Rule System:
d20/OGL 3rd Edition

From the publisher's description:
"The first book in the Maduban Saga, Leda centers on a young elven arcane apprentice in a human city who discovers adventure comes in many varieties. With the help of her friends and the reader's choices, she may succeed in staying alive, killing the bad guys, taking their treasure, and telling "stories" about how she was the heroine."
...
"If you're familiar with the d20 system, the resolution methods will be intuitive. If you're not, don't worry. You don't need to know how to play d20 games to enjoy a Party of One book. Simply roll the dice as indicated.
For d20 mechanic savvy readers, you'll occasionally be given a choice to run a combat yourself. Choosing to do so will take you to a combat appendix where creature stat blocks and a starting position map set up the action. All combat appendices end with options that bring you back into the story."