Author here. Several ways: the first half of the book was much more difficult to write as I was finding my voice and writing about non-Symfony tech (you can read the first two chapters on the website).

Additionally, I run my own software company so I can dedicate a full day to writing if I want (I've been writing in the evenings only).

Finally, if I don't feel I can make the date then I will let everyone on the pre-order list know and they can opt to have their pre-order canceled (no money exchanges hands until the book is published).

Thanks for the question!