It depends, Lin, on the type of book (as I think I suggested). If it's a fast-paced actioner, it's going to slow things down if you have to leave the action to look things up. If, however, the pace is slower, it won't have as big an effect.
Frankly, almost anything, when used inappropriately, can undermine the reading experience; which is why an author should try to understand what they're asking the reader to do at any given point in the narrative. (For example, a fight is not the best place to mention half a dozen new creatures whose characteristcs are to be found only in a glossary.)
I'm not a writing teacher, but thanks (
). I'm a telecoms consultant and, more importantly, a reader, a reader who doesn't like to be messed about because an author doesn't know what he or she is doing.