Would you prefer to use books or any hard copy references, or just the internet?

In my opinion neither stand alone. Since books are enrich with depth knowledge about any topic so I would like to refer it as divine water of river. Whereas internet is like sea you will get all the information on one point but there is debt about reliability since many times you are unaware about the source. Even though books are reliable but it has confinement where as internet has not.  Also, books will not provide you update information about current scenario of market, technology etc. which can be avail from internet. In today's world you have to depend on both then only you can survive the race.

It depends on the question you are trying to answer through research. Also, it depends on when in the timeline of an event, you are researching. However, in general, books are more valid since they go through the publishing process to check their credibility, while on the Internet valid sources are a few in comparison with opinionated pieces.

Internet is much easier nowadays.