Find the phrase to Tirth's strongbox, then return to Tirth.
I can help you with the book, but first you have to help me!
The tome you seek is in my magically locked strongbox. To open it a special phrase must be uttered, and I can't remember the phrase! Normally my assistant helps me remember these things but he's nowhere to be found.
If you want my book, then find my apprentice! Ask around the race track--if you're lucky then someone has seen him. Either drag him back here, or at least get my magic phrase from him so we can open my box!