Find Book Metrics
The traditional way to assess the importance of a book was to consider the reputation of its author, of its author's institutional affilation, of its publisher, and the reviews, venues, and reputations of its reviewers. This, of course, is all very qualitative.
Quantitative measures typically involve counting citations, and organizations are starting to put more effort into tracking citations to books and book chapters. Below are some databases and platforms that do count citations to books and book chapters.
Don't forget book reviews! Book reviews in academic outlets not only discuss quality, but place the book and its author in relationship to their discipline. Many academic journals have book review columns. Some disciplines have publications devoted to book reviews. Below are some examples.