Hi, ersott -- Well, I know nothing about the Neo Bebook, but we have two Kindles in our household, the DXi and the K2i. In each, the dictionary is built-in (you can choose from several, but the New Oxford American Dictionary is the default). All you have to do is put the cursor beside the word you are interested in, using the Kindle's five-way button, and the dictionary automatically pulls up the definition for you at the bottom of the screen. If you need to see more, you toggle the button to the right.
Finding the definition to another word? Just cursor to it. It's a great function! Battery life on the DX (the larger Kindle) is quite good, once it's stopped indexing the books you've downloaded (and that depends on how many it has to index at once). And you can charge it any time, as there is no memory effect. I usually plug mine in when it gets down to about half-power. That can be a week, depending how often I'm downloading. The smaller Kindle uses less power.
Oh, and having a dictionary definition doesn't interfere with reading your book, no!
Hope this helps!
Lesley
PS: I'm in western Canada; it's only 1:16 AM here....