Factual error: Zippo lighters are sold dry, no fluid when new.
Factual error: The film is set on Halloween (31st October) 1993. At the start of the film, Max is shown at school on Halloween. However, Halloween 1993 was actually a Sunday.
Factual error: The Sanderson House had been turned into a museum complete with a gift counter. The museum had been closed for years (note the dust and spider webs). The fluid in the lighter Max gets would have evaporated after a few months as Zippos are not airtight like a plastic disposable lighter would be.
Answer: They were set up, beforehand, just in case. Winnifred knew about the spell, but she didn't memorize the incantation. So she needed the book to see how to cast it.
Phixius ★
If you pause the movie, you will see that the book opened up to the Black Flame Candle spell.