After Processing Resources

Hi all - I'm a professional graphic designer - newbie/amateur/hobby photog. Strangely enough - even though I already own Photoshop for my pro work, I've never used it for after processing of photographs I've taken - that is unless it is something I am using in a layout.

So anyway, I am familiar with Photoshop - not with Lightroom - and I don't own it yet. Is Lightroom necessary to accompany Photoshop? Or is it a one or the other type of deal? What benefits can one achieve by owning both?

Also - is there some sort of "guide" or resource available which would help me learn a bit about what kind of things I might do with my photos during this after processing period? If you have any favorite books or web sites I'd really appreciate the information.