If i give you a simple difference
1. 읽어냈다-(" Oh Shit I did! I did read this book until the end!!holimoli!!) It can be an expression that you achieve some goals. in this case, you finish reading a book.
2.읽어버렸다.- "Hey man...I finished reading a book you recommended. I have nothing to read now..." It can be an expression that you did something and feel kind of regretting of doing it