Here is a list of books I've found that people refer to as "indispensable" when it comes to learning to write poetry.
Below that is a list of web addresses that I've collected over time that I've found interesting and helpful when I'm writing.
The Triggering Town: Lectures and Essays on Poetry and Writing
by Richard Hugo.
In the Palm of Your Hand: The Poet's Portable Workshop
by Steve Kowit
How to Read a Poem
by Edward Hirsch
A Poetry Handbook
by Mary Oliver
Sound and Sense
by Laurence Perrine and Thomas R. Arp
The Poet's Companion : A Guide to the Pleasures of Writing Poetry
by Dorianne Laux and Kim Addonizio
Links to Aid in Writing:
For they Rhymer
(searchable rhymes and synonyms )
Writing Publishable Poetry
Searchable listing of synonyms, synonyms and definitions
Common Writing Mistakes by Michael LaRocca
An editor’s view of the most common mistakes
Help File: Common Mistakes
Opinions of various writers and editors about their ‘pet-peeves’
Poetry Writing Tips