For very good political reasons, Booklab does not dispense tenure advice. The first of these is a practical one -- there are too many different departments with varying standards for any individual outside of those departments to be able to guess what constellation of publications will work for a tenure or promotion bid. The second is more subjective: tenure advice is really too much pressure to put on an individual or an office. After all, what if a professor were to ask for tenure advice, and then I gave it and was wrong? Does that mean the botched tenure bid is (gulp) my responsibility? Those are just two of probably many good reasons for Booklab staying out of the tenure advising business.
Therefore what follows is NOT tenure advice, but rather a definition of how some people have interpreted the variable statement "book plus." It often means a pattern of publishing over time where books -- if they are desired in the discipline -- are accompanied by a penumbra of articles both before and after the appearance of the book. Ideally these articles will be artfully spaced over time, but the author does not always have control over this because different journals have different incubation periods (anything from 6-8 months to as much as a year and a half, and very occasionally even more).
Then there is another competing meaning of "book plus" that involves one published book and significant progress toward a second one. I have heard faculty assert vociferously that the only correct meaning of "book plus" is one or another of these. Whichever one is used, however, it is generally done so in the context of distinguishing the university or department in question from what is known as a "one book" institution, meaning a place where you can get tenure for a book on its own, without accompanying articles or progress on a second book.
So what does your university mean when it uses the term? Have you heard another interpretation of the term "book plus"?