Is it an offence to ask a professor to share a soft copy of his recent book with me for my semester?
As an author I'd feel uncomfortable when someone asks me to share a digital copy of my book. I find it very much different from sharing digital copies of papers. An author of a journal paper gets these days a digital offprint (basically a pdf file), which one can share privately with colleagues. However as an author of a book you get (at least in my discipline) several free hard copies, which one can distribute among the dearest colleagues, but as their number is finite one may not be not always willing to share these with students. Sharing digital copies (which can be the technically last proofs the author gets before the book is printed) of the book may be a breach of the contract the author signed with the publisher, and one would feel uneasy if one has to choose whether to break the terms of the contract or to say no to a nice student interested in one's work.
So if I were you (and as I myself do when I miss a book written by someone I know well) I'd try one of the following: 1) request that your library purchase a digital (or print) copy of the book; 2) order the book via interlibrary loan; 3) buy the book. Otherwise you may send the author a nice letter saying that you are very much interested in their book and asking if they could advise you how to get hold this hard to find publication. (But again if you are at a university that has a library with access to ILL it would imply that you find it easier to trouble the author rather than order the book via ILL; hence I'd resort to this solution only if you are at an institution that doesn't provide other access options [which is surely the case in many countries with undeveloped library systems], and your request would look more reasonable if you explain in more detail why you cannot get the book without the author's help).