Ask clarifications to a professor of another university
Yes, you can certainly ask.  But here are some suggestions to increase your chances of getting a useful answer.


Before contacting the author, make a diligent effort to resolve the question for yourself.  Check other books or resources that you think might have the answer.  If there is someone else at your institution who is an expert in this area, consider asking them.  If there is an active StackExchange site for your field, consider asking there!
Put some effort into writing a question that clearly and concisely expresses what you are asking, and that can be easily understood and answered with a minimum of effort.  If the author can't understand what you are asking, or it is extremely long or complicated, she might not be willing to invest a lot of time trying to figure it out.  If you are unsure whether it is clear enough, or whether the English is understandable, consider asking someone (such as your advisor) to proofread it.  (You might read through Raymond and Moen's essay "How To Ask Questions The Smart Way".)
If you are asking about something that you think may be an error in the author's work, it is safer to express it as "I don't understand this" instead of "I think you are wrong".  It will be less embarrassing if they turn out to have been right.
Write in a way that makes it clear that you aren't asking a homework question or anything like that.
In general, your chances of getting a useful response are better if you are asking about a more specialized work that has relatively few readers.  If it's a lower-level, introductory, or extremely popular text, consider looking for help elsewhere.
Be patient.  Professors are usually very busy, and responding to questions about their books is usually among their lowest priorities.  I would give them at least 3-4 weeks; if no response by then, you could send one followup email.  If you don't get a response to that, then it's likely they are too busy to respond or not interested: stop emailing them, and look elsewhere for help.