It was good to see Skylark again, one of the very few lady setters. Literary themes are one of her specialities and we were going to be hunting for a book title in corrected misprints of fifteen clues. All the other clues would have an extra letter in the word play and these would give us an instruction.
There were a few very generous clues (to PYRAMID, ROARED, NEEDLED, ROE, SEE, SIDLED, AXLE, for example) and to the top and bottom rows of the grid, ‘Mad chess nerds vie, start to advocate separations (13)’ which anagrammed to DISSEVERANCES with an extra H, and ‘Police inspector’s future involves cramp causing unease (13)’ giving us DIS and COMING around FORT – DISCOMFORTING, and an extra R, so that the grid filled fairly speedily.
THE MATING SEASON emerged as the text and Google produced a summary explaining how Bertie Wooster, impersonating (“alias”) his friend GUSSIE, involved himself in a series of hilarious love affairs and coped with a bevy of aunts. However, finding the instruction spelled out by extra letters was not so easy: REPLACE ALIAS WITH HERO’S USUAL NAME eventually emerged.
GUSSIE was there in the diagonal, ready to have his place taken by BERTIE (leaving real words), and we needed 20 more letters in five words to produce the phrase he used to describe his predicament. Google to the rescue again. We fed in SLAVERING which appeared in a parallel diagonal, and were given A DEN OF SLAVERING AUNTS. What delightful P G Wodehouse language!
What a lovely construction. But what about the alcohol? Skylark gave us ‘bottles’ since ‘Drug in American aged pilgrims’ bottles (8)’ producing AM PULL AE but that was a poor show. Fortunately NAppy for NIppy finally produced the ALE we were looking for. “Cheers Skylark” and thanks for an enjoyable solve.

Good fun – and yes, the language is delightful!
Thanks so much, Dash. The Mating Season made me laugh so much on re-reading it that I hoped I’d spread the joy by perhaps encouraging others to read / reread it.
Thanks to Andy Dean too.