Digital

1 - 9 = 9 digits , number 10 to number 99 are double digits , so 90*2 = 180 digits , next 100 to 200 number are 3 digits each, so 101 * 3 = 303 digits,
next 201 to 236 makes 36 * 3 = 108 digits

so total 9 + 180 + 303 + 108 = 600

From 1 to 9, there are 9 digits. From 10 to 99, there are 2(99-10+1) = 2(90) = 180. From 1 to 99, there are 9+180 = 189 digits which is far less than 600 so the book must have at least 100 pages. To see if the book has 1000 pages or more, we find the balance number of pages which is 600-189 = 411 pages.

We see that number of digits from 100 to 999 = 3(999-100+1)= 3(900) = 2700 digits which is way more than 411 so the number of pages must be 3-digit. In other words, all the remaining 411 digits must come from 3-digit numbers so 411/3 = 137 3-digit numbers. So total number of pages = 137+100-1 = 236.