Back in the 1990s I worked a fair bit with Endnote. We tracked thousands of books and articles.
Since 2012 I’ve been working with two of the big four: Mendeley and Zotero.

Both are free. Both available for Windows, Mac and Linux. Both work with MS Word.
Mendeley is the darling right now, a NEW start-up from England. Very friendly.
But there’s a problem.
A gigantic publishing company Elsevier bought the start-up a couple of months ago, claiming only good intentions.

The news was not well received by librarians and scholars. Here’s their thinking …

Evil, greedy, unethical Elsevier will ruin Mendeley.
They might be right. Therefore, myself and many others are organizing to move to Zotero.
It’s free, open source, developed by the Center for History and New Media at George Mason University in Virginia.
I recommend Zotero.