Well, that's the thing... some authors are very transparent about their agenda. It only took one mystery novel for me to know where Sellers stood politically.
Yet her novel was well-executed.
John Irving is one of the few who really alientated me, eventually, though.
I knew where he was politically, based on the first novel I read by him. (World According to Garp.) However, I stuck with reading his earlier stuff, through Hotel New Hampshire, and even A Prayer for Owen Meany.
But Owen Meany was what soured me, because he became too transparent and preachy. Weave a world view into a compelling storyline, and I'm fine. But Prayer for Owen Meany went on long, 20-page political jags that did nothing to advance the story, it was just the author venting on politics... I tolerated it because there was a moving story of self-sacrifice buried under all that jangle... but it also forced me to just stop reading him. I wasn't willing to do it anymore.
So by the time he reached Cider House Rules and all that came after it, like A Widow for a Year, it was with one less reader willing to go for the ride with him.
But generally, unless it's really transparent like that, I'll stick it out if the novel's well-written.