Basically, the entire contents of the Legend Core Rulebook and all of the XXXXX of Legend are released as Open Game Content. This includes Pirates of Legend, Samurai of Legend, Gladiators of Legend, the Arcania of Legend books, etc. It explicitly
DOES NOT include anything from the Elric product line, the Deus Vult product line, or the Sheoloth product line.
To use this material, you have to comply with the terms of the Open Game License - which means that you need to attach a copy of the licence to your work and update Section 15 to list
ALL of the works that you used Open Game Content from. You also need to clearly identify what parts of your work are designated as Open Game Content and which parts are Product Identity. It's considered good form to give at least some Open Game Content of your own back to the community.
Also, if you want to indicate compatibility with the Legend game system and use the Legend logo on your work, you need to comply with the license available on the Mongoose site -
http://www.mongoosepublishing.com/pdf/leglogopack.zip
The key bit of text is:
The entire text of the Legend RPG is designated Open Game Content, as is the entire text of all books in this line with plain colour covers and the words ‘… of Legend’ in their title.
By using any Open Content material in any of these books, you also have permission to freely use the Legend Compatible logo on any publication or web site where this Open Content is used or modified. The Legend Compatible logo may be re-sized but may not otherwise be altered in any way.
Legend, the text of Legend rulebooks, the Legend logo and the Legend Compatible logo remain Copyright Mongoose Publishing 2011.
Any and all artwork included in Legend rulebooks is specifically not designated as Open Content, and may not be used without written permission from Mongoose Publishing.
The compatibility pack contains a copy of the logo for your use.
Note that while this license doesn't prohibit you from copying the "look and feel" of official Mongoose books, it is probably not something that Mongoose will want to encourage - they don't want too much confusion between what are "official" Legend rulebooks and what are third-party publications. As the previous poster indicated, check with Matt if you want to be on the safe side.