One writer I know carried her book idea with her for years and then, after those years, created an online class about the process of writing a book, just so she could hold herself accountable to finally writing her own. Today she's holding her book in her hands, and it's a work of beauty.
Another writer I know set a deadline for herself — her next birthday — to get her book proposal down on paper. She scheduled a conception session and then set about getting to work.
Still yet another writer decided last week — last week! — that after years of hindrances blocking her way, she was ready to commit to writing her book, investing in her process, and seeing it through to publication.
Sometimes it takes external factors. Sometimes it's an internal resolve. If you've been holding back on writing your book, what would it take to move you forward?