"Can you recommend a history textbook for 16 year olds about Western Expansion and the Civil War?"

And the sequel:
http://www.amazon.com/Behind-Blue-Gray-Soldiers-Readers/dp/0140383042/ref=pd_sim_b_title_1

Both these books are highly readable volumes that should whet the appetite of readers and inspire them to delve further into a this period.

Text uses letters, excerpts from journals and diaries, newspaper articles, and other primary source material to provide a look at life during the second half of the nineteenth century; when many Americans were moving westward.:
http://www.amazon.com/AmericanHeritage-American-Westward-Expansion-Heritage/dp/0471443948/ref=sr_1_2?ie=UTF8&s=books&qid=1211147526&sr=1-2

Text and maps tell the story of America's growth from 1754 to the early 1900s. Chapters are logically and chronologically grouped by appropriate eras with subdivisions providing access to regions or topics. Boxed inserts highlight people and inventions. The readable narrative, which makes up about half of the book, is suitable for students working on research papers.:
http://www.amazon.com/Atlas-Westward-Expansion-Alan-Wexler/dp/0816032068/ref=sr_1_1?ie=UTF8&s=books&qid=1211147785&sr=1-1

great resources for Civil War:
http://www.decatur.lib.il.us/lincoln/amyabib.htm

one must additional read for western expansion:
Bury My Heart at Wounded Knee: An Indian History of the American West (Paperback)
http://www.amazon.com/Bury-My-Heart-Wounded-Knee/dp/0805066691/ref=sr_1_1?ie=UTF8&s=books&qid=1211148174&sr=1-1

best wishes,
Librarian50