Text of an average book is 100,000 letters;
I’m not sure where you’re getting that value. The low end of word count for a novel is 50,000. If we say the average word is only 5 characters, we’re looking at a quarter million letters and another 50,000 spaces for a short novel (200-250 pages). Throw in some more for punctuation and formatting, of course. If you’re a fan of big epic fantasy/sci-fi you’re probably closer to a million words.
Halting State was great. It actually took me a couple of chapters to realize it was all 2nd person. That’s the book that got me into Stross.