@tono1020: Could you try reading a sample from
this book using the smallest font and tell me if your Kindle craches/exits to Home (displays some error message, and you can't open/read that book anymore until you open another book, change font size to 5th or 6th Aa menu value) after trying to go to the next page after that first one at the start of each chapter.
It's not that books fault ie there is nothing wrong with CSS & XHTML code, the same thing happens on my PW when I read same type of book. That is the books that have chapters that start with long paragraphs of italics text that is smaller than the rest of "main body text" = crash and exit to Home on first "next page" event.