I am often faced with questions in areas which cross my boundaries of knowledge. For instance, as an engineer, sometimes I am needed to study the basics of Anatomy or something like that.
This might be a temporary interest or might be permanent (There is no way of telling beforehand)
During such times, there is an option:
Go for the "proper" textbooks used by the students in that field. (Say this one :: 960 pages) and study them well.
Get a Schaum's series, Demystified or For Dummies sort of book and get over with it. (Say this one :: 450 pages). Alternately, use this as an entry point for the "better" books.
My professors have always suggested against such books because they provide no motivation for the development of a certain concept but I see that as a valid point only it is your primary field of interest. If an engineer wants to know Statistics or Anatomy, I am slightly skeptical whether such grinding is necessary.
Are such books a good resource/entry point for a new subject?
Note: All of this is under the assumption that the student is within the confines of academia.
Those books are not only very useful, I would argue they are the best option in many cases, as they represent the best use of your time. You will find that, throughout your graduate studies, you will have to learn aspects of many different fields. In most cases, while it would be possible for you to embark on a thorough study I some other discipline, that would take weeks, if not months. These books allow you to quickly learn the basics, giving you a solid foundation of knowledge that you can expand with further study if necessary.
Edit: this applies to topics only ancillary to your main field of research; you should not use this book on your research topic itself, for the reasons outlined by your professor.