I can part with gardening books such as A Directory of Trees and Shrubs (I can look up the excellent www.rhs.org for this kind of information, but I'll keep Trees For Your Garden by Roy Lancaster, as I'm getting the benefit of his knowledge, experience and advice, not to mention the pleasure of his witty and entertaining writing. Likewise gardening books by Beth Chatto, the late Christopher Lloyd, Helen Dillon, Dan Pearson and others.
This principle, keeping what I think of as 'reference plus' must apply to other categories of reference books too.
another posting, another time - the downside of the web for reference material......