There is ALOT of information on this topic in the d20 publishers/e-publishing forums; you'd do well to read the stickied threads over there.
Nine Hands said:
Yep, you can do this, but be careful. If for some reason the open content you are copying becomes closed your product goes out of compliance.
Open content can't be closed later*. That's why the SRD is forever.
*Actually, this isn't quite true. WotC removed a few items from the SRD that were judged important to the identity of Dungeons & Dragons, AND were creations unique to D&D, including beholders, yuan-ti, illithids, and the proper names used in some spells (Bigby, Leomund, Tenser, etc.) They aren't likely to remove much more.
OTHER publishers can't close pre-existing OGC; not even their own.
You also have to update section 15 of your copy of the open gaming license with any copyright information from Open Content you are using. As an example AEG's Mercenaries book has some of the following:
Dungeons, Copyright 2000 Alderic Entertainment Group
Evil, Copyright 2001 Alderic Entertainment Group
Dragons, Copyright 2001 Alderic Entertainment Group
So even if you own the copyright, you must decare its us in section 15 AFAIK. Check other publishers as I have heard some bad things about OGL compliance in regards to AEG.
In other words...
You are required to include the Open Game License (OGL) with any Open Game Content (OGC) you distribute (distribute meaning offer publically, whether it be material you've created, or material someone else has created). You are required to add the copyright information of whatever you're distributing (Merak's Manual of Money, copyright 2003, Merak Spielman) to the S.15, AND include the complete S.15 from any OGC source you use, minus repetition. Many publishers (include WotC) have made errors in updating their S.15's, so it's always a good idea to backtrack to an original source if you can.
You must CLEARLY designate OGC (including any you copy from elsewhere) and Product Identity (which are things you claim as yours). Material that is neither OGC nor PI is commonly referred to as closed content. You cannot declare other people's OGC closed -- it MUST be open. It is commonly accepted that game mechanics, particularly those totally reliant on the SRD, must be open. This includes feats, spell mechanics and effects, monster statistics and the "Combat" section of a monster entry, stat blocks, racial abilities of a new player race, and class abilities.
Many publishers are lackadaisical about clearly identifying OGC; while AEG has problems with it's S.15's, it's also one of the better publishers when it comes to identifing OGC.
Slide over to the d20 publishers forums for more answers.
Cheers
Nell.