Best extra books for an otherwise Core-only Druid?

[Endarire]

Let's assume you start at ECL1.  You can only use Core and Tome of Battle.  You may be able to convince the GM to allow another source or two.  (Planar Shepherds are right out!)  What would you choose and why?

Monster Manual III has fleshrakers.

Lost Empires of Faerun has Greenbound creatures.

Serpent Kingdoms has venomfire.

Spell Compendium has vigor and more spells than I can recall at present.

Magic Item Compendium has Chronocharms and affordable items in general.

Other sources may have more.

[Halinn]

I'd go for the Spell Compendium. It expands the spell list by a lot (including for instance the Bite of the X line), which increases versatility. It's perhaps not as powerful as fleshrakers, venomfire and such things, but I generally prefer versatility to power (operating with a certain power minimum satisfied plenty by the core druid).

[sirpercival]

Agreed for Spell Compendium.  It's the most important other book for pretty much any spellcaster.  It has kelpstrand, too.