If a reference/book authored by your professor contains typos, how should you react? My professor gave me a module on history that contains several spelling and grammatical errors. Should I tell him about this or just ignore these mistakes?
For the textbook I've written, I appreciate any feedback, including typos, misformed sentences, vague formulations and of course errors. Some points to consider (partly contained in the other answers):


Wait until you have a reasonable list or, e. g., until a chapter is over, the whole book, or the course. 
Check if there is some errata available somewhere. Some authors maintain an errata on their webpage, sometimes the publisher hosts this, for some lecture notes I have a blog post. Only send the points that are not already in the errata. 
There is no need to ask if such a list would be welcome. I guess it is safe to assume that this is the case (but others seem to disagree - see the comment below). 
For other feedback than errors and typos (e. g.  feedback for formulations or the organization of the book) be prepared that your comments may not be taken into account for various reasons.


Oh, and for the lecture notes I reward bonus points for every mistake spotted and submitted by a student.