Is this unprofessional academic writing? How to cite it anyway?
If you cite the secondary source (the book written by the sort-of-careless Harvard professors), you are not at risk of plagiarism.  That does not mean it is great practice: finding a primary source or something close to it is always a good idea.  But you will not be at fault if you simply site the Harvard profs and take them at face value.

One thing you could do is find secondary "primary" sources to back up the profs.  Then you could site an idea like this:


  ... and that is why monkeys like to steal bananas from the Harvard library (Mr. Fancy Harvard Prof 1, Mr. Fancy Harvard Prof 2, 1999) (Mr. Honest Zoo-Keaper Who Interned at Harvard, 1998).


Wherein the "Mr. Honest Zoo-Keaper Who Interned at Harvard" is the primary source you found on your own, externally, who seems to back up the less-than-well-documented assertions of the illustrious "Mr. Fancy Harvard Prof 1" and "Mr. Fancy Harvard Prof 2".