Helm Inc is the distributor.
Newest currently offered is 2018.
$595
iirc, there is always a few years lag between model year and public manual availability -- but maybe they are now discontinued.
Last set I bought was bound in multiple volumes, about 8.5 x 11 pages, and filled about 2 feet on the shelf.
Covered 1500/2500/3500, silverado and sierra, and all engines and transmissions offered in the year the set was for.
https://www.helminc.com/helm/product2.asp?Make=GMC&Model=SIER&Year=2018&Category=1&class_2=GMC&mk=GMC&yr=2018&md=Sierra&dt=Shop%2FService+Information&module=&from=result&Style=helm&Sku=GMT18CKPU&itemtype=N