The Roaring Trumpet and The Magic of Mathematics, two of the novellas nominated for the 1941 Retro Hugos, are the first two entries in a series by L. Sprague de Camp and Fletcher Pratt
De Camp and Pratt collaborated on a series of comic fantasy adventures featuring Harold Shea, psychologist and occasional enchanter. In our universe, Shea works in a psychology institute attached to a hospital, and along with a few of his his colleagues, has developed an interest in what they call paraphysics - the theory that all the worlds of the imagination exist, and the key to moving between them is the ability to shift one's sensory awareness from one universe to another.
In The Roaring Trumpet, the first of Harold Shea's adventures, he plans a nice trip to Ancient Ireland, but instead winds up in the universe of Norse mythology, in the midst of Fimbulwinter, with Ragnarok just around the corner. At first, Shea doesn't realise that the laws of physics he knows don't work in this universe - and neither do his matches, his gun, or anything else he brought with him, but once he works out the basic laws of magic, he gains respect as a warlock, and helps the gods prepare for their final battle. His bewilderment in dealing with the gods, giants, trolls and other magical folk from Norse legend provides much of the comedic enjoyment, and in the end, it's a fun - if somewhat bloodthirsty - romp through a mythic winter wonderland.
The Mathematics of Magic, the second adventure of Sprague de Camp and Fletcher Pratt's Compleat Enchanter Harold Shea, takes place in the universe of Spenser's Gloriana, where Arthurian-style knights comport themselves nobly against a background of Elizabethan pageantry. This time Shea is accompanied by his colleague in psychology and "para physics," Dr. Reed Chalmers, as they roam from tests of arms to jousts and tournaments to battkes with evil magicians under the guidance of the great female knights Britomart and Belphebe, and the much-imperiled damsel Amoret, committing magical mayhem as they go. De Camp and Pratt offer a fine parody of the excesses of the courtly literary tradition, with a few trenchant comments on the general position of women in the world of knights and fair ladies.