To adjust the brightness just slide your finger up or down on the right side of the screen. That works for me on my Galaxy Tab.
I've been using the Mantano reader almost exclusively for about 3-4 weeks now and like many am very impressed with it.
My personal niggle's with it are the toolbar/menu bar at the bottom - there's a few too many options and the ones i use most commonly - TOC/Notes and Theme/Display - are at opposite end's of the toolbar which involves scrolling accross.
I've also noticed that it sometimes seems to lose track of where you are in a particular epub file - the library page shows the right page but it doesn;t open to there.
The only other issue is onr or two epubs that i've converted from mobi to epub via calibre have caused the app to crash. It restarts cleanly but totally fails for some reason on those files - aldiko handles them fine. I got around it by converting them via mobitools instead so not a big issue for me.
All in all though, an excellent reader with very good support and at least 4 updates in the short time i've had it - and a very well laid out PDF manual.
Well worth the money.