Black Library's quality can vary quiet widely, even their best authors have had the odd blip. Abnett's GG novel Salvations Reach and McNeill's recent Priests of Mars, while both enjoyable reads also left me disappointed for the same reasons, an undeserved premium price tag(The ebooks should not cost more because the paper copy is a hardback IMO) and dependancy on the sequel to get full enjoyment. I'm definitely more used to the Gaunt's Ghosts books feeling complete and self standing even though they have focussed on campaigns involving several books, SR felt unfinished.

Authors I would recommend to people who are wary of trying anything beyond Abnett & McNeill would be Aaron Dembski-Bowden, Chris Wraight & Rob Sanders. Of the Space Marine battles novels ADB's Helsreach was really enjoyable and apt for lovers of the third war for Armageddon; Wraight's Battle of the Fang and Wrath of Iron were good but the latter was very dark even by 40k standards.

Sander's Legion of the Damned whilst not strictly focussing on the Legion per se did enhance everything we think/know about them, as an Imperial Fists fan I particularly adored this book as it folows a new successor Chapter and fleshes out both the IF Legion and their successor's customs. Sander's Atlas Infernal is also worth a look for Inquisition/Inquisitor Czevak fans.