Unholy damage reduction in Book of Exalted Deeds
Book of Exalted Deeds uses both damage reduction that's overcome by evil weapons and DR that's overcome by unholy weapons. However, BE uses DR that's overcome by evil weapons frequently—over 30 times—and uses DR that's overcome by unholy weapons only four times: the supernatural ability blessing of the glittering heaven, a level 9 class feature of the prestige class celestial mystic (56); the extraordinary ability celestial transformation, a level 10 class feature of the prestige class initiate of Pistis Sophia (65); the extraordinary ability celestial ascension, a level 10 class feature of the prestige class troubadour of stars (80); and the DR 15/cold iron and unholy of the tulani, a new kind of eladrin (171—2).
BE does not feature any instance of DR that can only be overcome by a weapon that's both evil and unholy. Further, BE never explains how to overcome DR that's overcome by unholy weapons at all. Presumably the magic weapon special ability unholy (Dungeon Master's Guide 226) (+2 bonus; 0 lbs.) is supposed to overcome such DR, but even that's technically a guess as the text is silent.
That's enough for this DM to rule that DR that's overcome by unholy weapons is, in fact, DR that's overcome by evil weapons. BE's four instances of DR that's overcome by unholy weapons is not enough to convince this DM that the BE wants the DM to develop his own new and unique rules to cover for it omitting an explanation of DR that's overcome by unholy weapons… especially when perfectly serviceable rules for DR that's overcome by evil weapons already exist.
Unholy damage reduction in the core rules
I suspect those four instances of DR that's overcome by unholy weapons stem from one very particular misstatement. This requires some introduction. The Dungeon Master's Guide on Damage Reduction, in part, says that besides damage reduction that's overcome by weapons created from special materials
Other kinds of damage reduction are overcome by magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment (such as that granted by the holy special ability or the align weapon spell). (291, emphasis mine, and—as always—examples absent from the SRD)
No mention of DR that's overcome by unholy weapons. The Monster Manual on Damage Reduction, in part, says
Some monsters are vulnerable to chaotic-, evil-, good-, or lawful-aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature. (307—8, emphasis mine, and link added)
Also no mention of DR that's overcome by unholy weapons. However, the Monster Manual v.3.5 Errata on Damage Reduction offers no information on damage reduction that's overcome by evil or good weapons. Instead, the Errata, in part, says
Damage Reduction is either extraordinary (Ex) or supernatural (Su). Use the following guidelines if it is not specified.…
- DR X/chaotic, lawful, holy, or unholy should be (Su)[.] (1)
(Just to be clear, no creature in the Monster Manual actually possesses DR that's overcome by holy weapons or by unholy weapons. And, yes, much to my dismay, the premium edition of the Monster Manual (2012) incorporates this errata as-is.) Thus it appears that the Monster Manual errata accidentally conflates holy and good as well as unholy and evil. This reader suspects that the author of those four instances of DR that's overcome by unholy weapons in the BE was misled by the MM errata.