- Robin Hemley's article which had practically started it all
- An opinion column on it by Manuel Quezon III published in Inquirer.net
- The Philippine Genre Stories multiply page by Kenneth Yu. It's worth reading as he had kept his cool and posted Undersecretary Sales' side on the matter (scroll down to the comment posts).
- Florence Agreement (June 17, 1950) (Note that the Philippines had signed the agreement on Aug. 7, 1979)
- Presidential Decree No. 284: Amending Subsection (s) of Section 105 of R.A. 1937, Otherwise Known as the Tariff and Customs Code (Sept. 3, 1973)
- Presidential Decree No. 1464: A Decree to Consolidate and Codify All the Tariff And Custom Laws of the Philippines (June 11, 1978)
And I just have to quote this (Text enclosed in  and starts with MY NOTE are my insertions. Bold highlights are all mine.):
TARIFF AND CUSTOMS CODE OF THE PHILIPPINES, Sec. 105.
Conditionally-Free Importations. — The following articles shall be exempt from the payment of import duties upon compliance with the formalities prescribed in, or with, the regulations which shall be promulgated by the Commissioner of Customs with the approval of the Secretary of Finance... [MY NOTE: Three other provisions then follows]:
[MY NOTE: It then list specific items that can fall on this section. The one we are interested in is Subsection (s) of Section 105 which had been amended in 1973]
Section 1. Subsection (s) of Section 105 of Republic Act Numbered nineteen hundred thirty-seven, as amended, is hereby further amended to read as follows:
"s. Economic, technical, vocational, scientific, philosophical, historical, and cultural books and/or publications: Provided, That those which may have already been imported but pending release by the Bureau of Customs at the effectivity of this Decree may still enjoy the privilege herein provided upon certification by the Department of Education and Culture that such imported books and/or publications are for economic, technical, vocational, scientific, philosophical, historical or cultural purposes or that the same are educational, scientific or cultural materials covered by the International Agreement on Importation of Educational, Scientific and Cultural Materials signed by the President of the Philippine on August 2, 1952, or other agreements binding upon the Philippines.
"Educational, scientific and cultural materials covered by international agreements or commitments binding upon the Philippine Government so certified by the Department of Educational and Culture.
"Bibles, missals, prayer books, Koran, ahadith and other religious books of similar nature and extracts therefrom, hymnal and hymns from religious uses."
It might be how I'm reading it but I still can't find the specific provision for 5% or even the 1% duty they are imposing in the above section. In fact the way I'm reading it (and I might be wrong) books (provided they are "economic, technical, vocational, scientific, philosophical, historical, and cultural books" and also included books specified in Annex A of the Florence Agreement) should be excepted from any duty. In Kenneth Yu's interview of Undersecretary Sales, she had mentioned that the 1% is "for, to use her words, "control/monitoring" of the imported books coming in".My opinion?
It all comes down to their (Department of Customs or Finance?) implementation of their interpretation of Section 105 of the Tariffs and Customs Code versus the current implementation of Florence Agreement on the Importation of Educational, Scientific and Cultural Materials.
I'm interested in what the National Book Development Board will do. I rather wait for every possible stake holder's response before posting anything further.