Writing a book sounds impossible until you look at the numbers.
A 100-page book is about 30,000 words. If you write 1,000 words per day, you finish writing your book by the end of the month.
If you want a 200-page book, then writing 1,000 words per day gets the job done in 60 days.
Not everyone has time to write 1,000 words per day. However, even if you write 500 words per day, you can get a book done within a few months.
Just make sure you write an outline for your book before you start writing. Most people think it's impossible because they don't create an outline first.