Although David's painting is one of the most famous images of Napoleon, the campaign of 1800 is pretty underserved in the Napoleonic literature. This is partly because the campaign itself did not necessarily showcase Napoleon in the best light; his initial plan to go into Austria was foiled by developments in Italy that necessitated him to cross the Alps into northern Italy and Marengo was really Desaix's victory, not Napoleon's. The best coverage of the 1800 campaign is James Arnold's Marengo and Hohenlinden: Napoleon's Rise to Power which does have a section on the St. Bernard pass. The relevant sections of David Chandler's The Campaigns of Napoleon also detail the pass crossing as well as Napoleon's own strategic thinking. The Napoleon-series.org has a couple of good articles on the 1800 campaign.