How are you? I'm glad you liked it! ^^ I really can't give you an exact figure, but I can offer the following recommendations:
- Avoid using too many images, and try to keep them lightweight; under 100kb is advisable.
- Try not to use too many pages; it's better to have several books, but they shouldn't be extremely long, around 3-12 pages per book should work appropriately without issues.
In these cases, trial and error is often the best way to test it out. ^^