Lovell's Wikipedia entry is here, his Society of Illustrators Hall of Fame page is here, and two blog posts worth viewing are here and here.
He began by illustrating for "pulp" (cheap) magazines while still at Syracuse University in upstate New York. From pulps, he soon moved up to the prestigious and better-paying "slick" magazines and remained there for the rest of his illustration career.
Lovell characterized himself as a visual story teller (his pulp period was good training for that, he allowed) and researcher. Regarding the latter point, he felt that his duty was to get details right, and this required a good deal of preparation because many of his subjects were historical. Motivation for this almost surely was the fact that illustrations with incorrect details are criticism-fodder for sharp-eyed readers.
One observer has commented that Lovell's style didn't change much over his career. This seems to be generally true, though he clearly adjusted it to the requirements of the subject. On the other hand, Lovell's style was not as distinctive as those of some other top-notch illustrators. That is, a typical Lovell illustration is clearly very competently done, yet it can be difficult to instantly identify it as his work without searching for his signature.
A Marine Corps sergeant on Asiatic duties in the 1930s, I think. Painted by Lovell when he was in the Corps during World War 2.
That's Robert E. Lee, at the left, surrendering his army to Ulysses Grant (at the table to the right), effectively ending the American Civil War.