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.