When Brian Eno was working with U2 and Luciano Pavarotti on the song Miss Sarajevo, he observed that Pavarotti came to the studio, looked over the libretto and noted which lines were easiest to sing. He sang those first.
Instead of singing the operatic verse through, he did it in pieces.
Why wouldn’t he? The idea that he would sing the full verse in order and deliver one perfect performance is what the audience sees. It’s not how the work is created.
Those who start out writing a book, or any creative work, often think that this is the way it is meant to be done. The professional knows that it is done in pieces, and gets to work by first understanding and defining exactly what are those pieces.
Breaking the task into pieces you know how to tackle can save time. It can help maintain momentum. You can structure the work to suit your flow by placing more difficult parts where you know you will be most suited to tackle them.
What is the easiest next step in writing your book? Do that first.