style/Format question

[Mandy]

I'm both learning a lot from and very interested in this book. It is very well written, but I'm confused. Why are quotations started, "[W]ith first letter in brackets (like this one)?" Does anyone know what format this is and or what it means?

[Jerome]

I'm pretty sure whenever this is done, the original document the author is quoting from was poorly written, with words misspelled and letters left out.