I highly recommend the book. It's fantastic--one of the most gripping pieces of nonfiction I've read in a long time--and will help you weed out what was true in the movie from what was "Hollywooded." In general, the movie was fairly accurate on the big stuff--i.e., it gave a pretty realistic overall impression of Seabiscuit and the people who made him famous--but compressed certain incidents/characters as necessary in order to keep the plot (and length) from getting unwieldy.
And FYI, Seabiscuit's owner was not the founder of Buick, just an early Buick dealer/entrepreneur who made his fortune by becoming the sole Buick distributor for the western U.S.
Common sense is not so common. -Voltaire