Start with Professor O’Toole’s own:

When Trumpets Call: Theodore Roosevelt After the White House. New York: Simon & Schuster, c2005.

Other good books on the Progressive Party and the 1912 campaign are:

Broderick, Francis L. Progressivism At Risk: Electing A President In 1912. New York: Greenwood Press, 1989.

Chace, James. 1912: Wilson, Roosevelt, Taft & Debs-- The Election That Changed The Country. New York: Simon & Schuster, c2004.

Flehinger, Brett. The 1912 Election and the Power of Progressivism: A Brief History with Documents. Boston: Bedford/St. Martin's, c2003.

Gould, Lewis L. Four Hats In The Ring: The 1912 Election And The Birth Of Modern American Politics. Lawrence: University Press of Kansas, c2008.

Mowry, George Edwin. Theodore Roosevelt And The Progressive Movement. Madison: The University of Wisconsin Press, 1947. Sixty years old, but still valuable.

For the influence of the rising use of primary elections:

Ware, Alan. The American Direct Primary: Party Institutionalization And Transformation In The North. New York: Cambridge University Press, 2002.

One of the liveliest Progressives was undoubtedly Robert LaFollette of Wisconsin. These books provide excellent background:

Unger, Nancy C. Fighting Bob La Follette: The Righteous Reformer. Chapel Hill: University of North Carolina Press, c2000.

Weisberger, Bernard A. The La Follettes Of Wisconsin: Love And Politics In Progressive America. Madison: University of Wisconsin Press, c1994.

Roosevelt’s running mate, Hiram Johnson, was almost as lively and interesting a figure as Teddy himself:

Johnson, Hiram. The Diary Letters Of Hiram Johnson, 1917-1945. Introd. by Robert E. Burke. New York: Garland, 1983.

Lower, Richard Coke. A Bloc Of One: The Political Career Of Hiram W. Johnson. Stanford, Calif.: Stanford University Press, 1993.

Olin, Spencer C. California's Prodigal Son: Hiram Johnson And The Progressives, 1911-1917. Berkeley, University of California Press, 1968.

The Bull Moose Party wasn’t the only force in the 1912 race. For Taft and the Republicans, take a look at:

Anderson, Judith Icke. William Howard Taft, An Intimate History. New York: Norton, c1981.

Burton, David Henry. William Howard Taft: Confident Peacemaker. New York: Fordham University Press, c2004.

Wilensky, Norman M. Conservatives In The Progressive Era: The Taft Republicans Of 1912. Gainesville, University of Florida Press, 1965.

For a refresher course on Woodrow Wilson and the 1912 race:

Cooper, John Milton. The Warrior And The Priest: Woodrow Wilson And Theodore Roosevelt. Cambridge, Mass.: Belknap Press of Harvard University Press, 1983.

Remember that Eugene Debs, candidate of the Socialist Party, garnered almost a million popular votes in 1912:

Ginger, Ray. The Bending Cross: A Biography Of Eugene Victor Debs. New Brunswick, Rutgers University Press, 1949. This is old, but very readable.

Morgan, H. Wayne (1962). Eugene V. Debs: Socialist for President. Syracuse: Syracuse University Press, 1962.

Salvatore, Nick. Eugene V. Debs: Citizen And Socialist. Urbana: University of Illinois Press, c2007. The most recent Debs biography, but a bit scholarly for some tastes.

The role of African Americans in the Progressive movement and the 1912 Bull Moose campaign is fascinating and tragic. This book gives a good overview:

Southern, David W. The Progressive Era And Race: Reaction And Reform, 1900-1917. Wheeling, Ill.: Harlan Davidson, c2005.

This study delineates the reimposition of segregation in Washington under Wilson:

Patler, Nicholas. Jim Crow And The Wilson Administration: Protesting Federal Segregation In The Early Twentieth Century. Boulder: University Press of Colorado, c2004.

Internet:

The Time/CNN website on Roosevelt, has a good “War of 1912” segment:
http://www.time.com/time/magazine/article/0,9171,1207791-1,00.html

Wikipedia’s entry is solid:
http://en.wikipedia.org/wiki/United_States_presidential_election,_1912

Ourcampaigns. com has a nice segment on the role of primary elections in 1912:
http://www.ourcampaigns.com/RaceDetail.html?RaceID=55164

National Archives “Teaching with Documents” has a lesson plan on the political cartoons produced by the 1912 race:
http://www.archives.gov/education/lessons/election-cartoons/

Harpweek’s “Explore History” section has terrific materials on “Presidential Elections, 1860-1912”. For each election, you’ll find a good analytical “Overview,” cartoons, biographies of candidates, and “Events” chronology. The sections on 1900 and 1904, of course, give valuable materials on Roosevelt’s race as Vice President and his first race for the Presidency. The 1912 election inspired some great material. Don’t miss this one:
http://elections.harpweek.com/

Another good site on the election comes from Ohio State University:
http://ehistory.osu.edu/osu/mmh/1912/

Miller Center of Public Affairs at the University of Virginia has the "American President: An Online Reference Resource" website that provides good basic information, recommendations for further reading, for Roosevelt, Taft, and Wilson:
http://millercenter.org/academic/americanpresident

The PBS American Experience episode on Woodrow Wilson is rich in resources. Here’s a good interactive game on the 1912 race website updated in 2001:
http://www.pbs.org/wgbh/amex/wilson/sfeature/sf_election.html

as well as a lesson plan on the election:
http://www.pbs.org/wgbh/amex/wilson/tguide/t_lesson_03.html

and on Wilson and African Americans:
http://www.pbs.org/wgbh/amex/wilson/tguide/t_lesson_02.html

And “Further Reading” good articles and links to Internet sources for Wilson:
http://www.pbs.org/wgbh/amex/wilson/filmmore/fr.html

American Memory gives sound recordings and transcriptions of T.R. speeches in 1912 Bull Moose campaign:
http://memory.loc.gov/ammem/collections/troosevelt_film/trfsnd.html

The Wisconsin Historical Society website has a very helpful page on LaFollette – biographical sketch, links to documents and other primary sources, links to other websites and recommendations for books and articles. Not a real surprise that this Historical Society would do such a good job, but I always enjoy seeing work of this quality on the web:
http://www.wisconsinhistory.org/turningpoints/tp-035/

Less expectedly, the Historical Society also gives us a very nice piece on Roosevelt’s brush with death in Milwaukee October 14, 1912:
http://www.wisconsinhistory.org/museum/artifacts/archives/001692.asp

Library of Congress Virtual Programs and Services “Web Guides” are a great innovation. You’ll find a guide to materials in LC collections and in other Web-based publications. Bookmark this one as a “Favorite” and keep checking back to see future additions:
http://www.loc.gov/rr/program/bib/bibguide.html

The “Presidents” section, at present, gives materials only for Wilson, but stay tuned as they update the service and add materials for Roosevelt and Taft:
http://www.loc.gov/rr/program/bib/presidents/index.html

The Eugene Debs Foundation has done a great job on their website:
http://www.eugenevdebs.com/

I especially recommend the website for the Indiana State University (Terre Haute) Library’s Debs Collection which I discovered thanks to the Debs Foundation links– most of the listings are for catalogs of their holdings, but scroll down to photos and videos:
http://library.indstate.edu/about/units/rbsc/debs/debs-idx.html