When asked via e-mail about CLAMP in America guidebook, publisher DelRey replied that the title has been cancelled:

The title ‘Clamp in America’ has been cancelled. There is no tentative release date scheduled for this title at this time. The only pending volumes currently scheduled for release from Clamp are XXXHolic 17 in September of this year and XXXHolic 18 in December of this year. If you would like to write to the Del Rey editorial team to request they reconsider, please do so at this address:
Random House
c/o Del Rey Editorial Department
1745 Broadway
New York, NY 10019

Thank you,
Consumer Services
TU-863809

Thank you syaoranmew for sending the e-mail to DelRey and sharing their reply!

Such sad and unfortunate news =/ after being delayed over and over again, it was cancelled in the end. Not surprising, but still very sad.