Is it frowned upon to request solutions manuals from the author? Maybe the answer to this is probably as vague as "it depends." I'm a graduate student trying to pick up some extra math skills. I'm reading a couple of texts and attempting the exercises but the difficulty/specificity of some exercises is such that I fear somewhat frequently asking for guidance would be burdensome to whoever I ask for help (peers, professors, my advisor, etc.). My preferred system would be to try my hand at a collection of problems for a few days, and check my thinking against a solutions manual (consulting others if I encounter something I really don't understand). 

Some authors encourage instructors to contact them for solutions manuals if they are planning on teaching with their book. As a self-studier, should I generally expect a similar offer extends to me, or would it be frowned upon to ask for a solutions manual? 

Thanks in advance for any help!
Your request, as a self-studier, to obtain the solutions manual seems perfectly reasonable, and I wouldn't hesitate to make it.

It's true that your request might not be successful, because, as some commenters suggested, the solutions manual may be kept away from students to prevent cheating.  It may be hard for them to verify that you are not a student trying to cheat.

However, I see no harm in asking.