The thought proces is: let sell out the LLC on open market and distribute net proceeds after deducting all costs and taxes. For those departing partners, this is fair for them to incur their portion of taxes and transaction costs (potentially plus additional compensation to remaining partners for "lose" in such a "premature wash sale"). For remaining partners, they simply "defer" realizing gains and taxes, and hence carrying normal risks and benefits of such deferring.
Book value is to Uncle Sam for LLC tax purposes and is independent of negotiation among partners on partner changes.