Mantano is good, but I do have a couple of small issues with it.

One, like most of the 3rd party reader apps, it tends to take too many presses to bring up a definition in the dictionary. In the stock Fire reader you simply long press to select a word and the dictionary popup is displayed with a short definition. in Mantano you long press to attempt to select a word, but 9 times out of ten it will select anything but your word as the selector seems very buggie. Once the word is selected you then press the Dictionary button at the bottom of the screen to popup a window then from that you press the magnifying glass icon to actually bring up the definition. This is way to kluge and you have to move your finger about on the screen. Compare the Fire stock reader where it is simple and elegant, and your finger can stay near the word you looked up.

Two, you can sideload fonts, but they often display horribly wrong making them worthless to use. I sideloaded Caecilia, Lexia DaMa, and some others but they all display with the bottom alignment of the letters askewed. It is difficult to read words where the letters are at different heights above the bottom line.