Castle Garden (www.castlegarden.org) has greatly expanded its webpage to include “information on 11 million immigrants from 1820 through 1892, when Ellis Island opened.”
Library and Archives Canada (www.collectionscanada.gc.ca/genealogy) has useful information about immigration to and citizenship in Canada as well as links to passenger lists both before 1865 and from 1865-1935.
Genealogical webpages of interest for Schleswig-Holstein include the following: Hans-Peter Voss’s page (www.genealogy-sh.com); ProGenealogists, a professional research firm connected with Ancestry, has several useful pages including a S-H place name index and reverse index at www.progenealogists.com/germany/schleswig/fschlhol.htm); Family Search offers 96 sub-pages of information at http://FamilySearch.org/learn/wiki/en/Category:Schleswig-Holstein; the German group AKVZ (Census Records Research Group) has several databases with early censuses and other information, mostly in German at www.akvz.de/index.html. Access to the various databases may require a fee; however, its’ worth taking a look at what has been digitized or indexed. Included in their area of interest are also the duchy of Lauenburg (which was given to Denmark in 1814 in exchange for Norway), the principality of Lübeck, and the Grand Duchy of Mecklenburg-Schwerin. These latter would be natural area of migration into S-H, and later, into Denmark.
An enlightening article on what happened to American-born women who married non-citizen immigrants following the Expatriation Act of 1907 and related provisions of the act may be found here: http://blog.mocavo.com/2014/04/revoking-american-citizenship-early-twentieth-century?