Can this verb be used as in English (e.g. 'you have the guide-book; what does it say?'), or is it only for things that are literally speaking?
Nope, that would be "mitä siinä lukee?", so it's more like "what does it read?". Note the use of inessive case in addition to the different verb.
Thanks. I suspected this might be the case but wasn't sure.
Now I'm looking at inflection tables of Finnish pronouns and appreciating how easy Japanese really is!
Or one could ask "Mitä siinä sanotaan?" [like, "what's being said in it"].
Or "mitä siinä seisoo?" Like in Swedish "vad står det?"