How large is the largest known prime number?

In editing Mathematics Explained for Primary Teachers I had to update the bit about the largest known prime number, because clever people with even cleverer computers keep discovering larger ones. So far every new edition of the book has required an update! 

At the time of writing, to my knowledge, the largest known prime number was ‘2^57,885,161 − 1’. This means 57,885,161 twos multiplied together, minus 1: which produces a number with over 17 million digits.

How large is this? It's very difficult to conceive of a number with 17 million digits. So, I came up with the following idea. Imagine trying to write it down. The fifth edition of my book will contain over 400 pages. Just to print all 17 million-plus digits in the largest known prime number would require ten books of that size!