The contents is clearly marked public domain and free to use. I would like to publish it as hard copy and include further original content along with it. Can I copyright the resulting book? Is there some ratio of original content to public domain content that must be met before it can be copyrighted?

All Answers To Questions

According to this there is a original/copied ratio...'to be determined by the courts'. So you're best bet is to contact a copyright lawyer and talk to him. 6. Sec. 107. Limitations on Exclusive Rights: Fair Use In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include: the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; the nature of the copyrighted work; the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the effect of the use upon the potential market for or value of the copyrighted work. The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors. Comment: Section 107 is a statute within a statute. The important point about the section is that it grants rights to users and is to be interpreted accordingly. Arguably, without section 107, the 1976 Copyright Act would be unconstitutional in light of the limitations in the copyright clause on Congress' power to enact copyright legislation. Thus, without section 107, section 106(1) would give the copyright owner the absolute right to copy, which would mean that copyright could be used to inhibit rather to promote learning.

It sounds like you're looking to protect a derivative work. Here's what the US Copyright Office has to say: "A “derivative work,” that is, a work that is based on (or derived from) one or more already existing works, is copyrightable if it includes what the copyright law calls an “original work of authorship.” Derivative works, also known as “new versions,” include such works as translations, musical arrangements, dramatizations, fictionalizations, art reproductions, and condensations. Any work in which the editorial revisions, annotations, elaborations, or other modifications represent, as a whole, an original work of authorship is a “derivative work” or “new version.” A typical example of a derivative work received for registration in the Copyright Office is one that is primarily a new work but incorporates some previously published material. This previously published material makes the work a derivative work under the copyright law." Hope that helps!