You can find the Maus tank in almost any German Secret Weapons book, the old Schiffer series, or any concise book on tank history or armored vehicles of Nazi Germany/Third Reich.
The Rammtiger AFAIK does not have its own book since the type is rare (an odd PzKpfw VI Tiger-P variant) and only 3 were ever supposedly converted over. You may find mention of this in a German armor of WW2 book.
Try using this site, www.bookfinder.com and enter Maus tank and Rammtiger into the title field. See what comes up. Otherwise, seek out your local hobby shops that carry military books or your local bookstore in the war section.