Yes! There is only ONE WAY to change the screen brightness below the NC system's lowest setting while reading though. As @Bookworm_Girl stated, "Open your book in the reader. Tap in the middle of the screen. A menu window pops up at the bottom." On the right side of this popup menu is a brightness control item, click it and use the popup slider bar to lower the brightness below the system default low.

There are two other brightness control menus, but they only adjust the overall system brightness. Use the menu listed above to lower the brightness even lower for books only.

BTW, if you know how to strip the DRM and use calibre to convert/modify ePubs, you can change all text to BOLD via modifying the CSS. It is rather complicated so I won't cover the technique here, but search in the MobileRead forums for instructions. I modify all my ePubs to use BOLD text because my eyes are not that good either. I find that the bold text helps significantly for both the NC with its LCD as well as eInk screens.