Be very interested to hear what other folks here have to say about this, but my read of the RAW is that you got it right for Stubborn, and Books of St. Lucius are a little more powerful. For everybody's reference, Weaken Resolve says:
The psykers reach out with their minds, evoking irrational terror in the minds of their foes.
This power is used during the Psyker Battle Squad's Shooting phase. Choose one enemy unit within 36" and line of sight of the Psyker Battle Squad. For the remainder of the turn, the enemy unit's Leadership is reduced by the number of Sanctioned Psykers in the unit utilising the power (to a minimum of 2).
Stubborn says:
Stubborn resistance against impossible odds is a feature of some races. When taking Morale tests, stubborn units always ignore any negative Leadership modifiers. Independent characters that are stubborn confer the ability onto any unit that they join.
And the Book of St. Lucius says:
This great tome contains the many inspirational writings and sermons of St. Lucius of Agatha. Any friendly unit with a model within 6" may use the bearer's unmodified Leadership value for any Morale checks or Pinning tests it is required to take.
The question, as I see it, is whether Weaken Resolve "modifies" the target unit's Leadership, or does something else to it. My opinion (though again I'm interested in others') is that it modifies the unit's Leadership, and thus is affected by Stubborn and Books of St. Lucius, for the following reasons:
1. My personal feeling is that since "modify" is nowhere defined in the rulebook, it should be given its ordinary meaning. Weaken Resolve causes a unit to be treated as temporarily having a Leadership value other than what is printed in its codex. This seems to me, on a plain reading, to be a [url=http://dictionary.reference.com/browse/modify]modification[/url] ("to change somewhat the form or qualities of; alter partially; amend").
2. Some pieces of wargear "modify" a characteristic for the entire game, yet are still classified as modifiers. A scorpion chainsword's +1S, for instance, affects the model for the entire game, and yet is still classified as a modification (p. 7). On the Leadership front, a Book of St. Lucius pretty clearly would not apply to a Holy Icon, which adds 1 to a unit's Leadership value for purposes of Pinning and Morale tests. Surely the +1Ld from the Holy Icon would be considered a Leadership modification, yes? Yet a Holy Icon potentially affects a unit (e.g., a Retributor squad that spends the game next to its Rhino or Immolator) for the entire game, not just a single turn. A fortiori, Weaken Resolve, which lasts for only a turn, is a modification as well.
3. The most tempting alternative reading I can think of is to say that Weaken Resolve somehow temporarily overwrites the unit's printed Leadership, and thus does not modify it because the rule instructs us to pretend, for a turn, that the codex says something other than what our eyes tell us it says. I see no reason why the power couldn't say this, but it would have to spell it out pretty explicitly, since unless you word this pretty precisely (or else make a defined-term distinction between "modifying" and "overwriting") most ways of achieving this effect boil down, in plain language terms, to modifying.
EDIT:
Weaken Resolve does work on Stubborn units, and on units with a Book of St. Lucius, just not all the way. Stubborn doesn't help with Pinning, for instance, or with psychic tests (if there were such a thing as a Stubborn psyker). Weaken Resolve would still affect a Stubborn unit in those circumstances. Similarly, a Book of St. Lucius only protects a unit from Weaken Resolve with regard to Morale and Pinning tests; any other sort of Leadership test would still be affected by Weaken Resolve.