The problem is that I know too much about history. Certain things grate on me, anachronisms, mostly, but other things as well.
It's worse when a novel has lulled me into false security. I am reading one like that currently. Wow, she did her research. A lot of items are right out of original source documentation. And then she screws it up. Screws it up so badly that I want to throw the book across the room. But I know she's not the first, and won't be the last. But I do so want to read novels that go so smoothly that you forgive a few flaws.
Here are a few pointers for keeping your reader happy.
Don't put in plants that shouldn't be there. I know it's cute to introduce plants you're familiar with, but don't put a poinsettia into a 19th century Christmas, as I've seen done. True, Poinsett brought them back from Mexico in 1823, but it wasn't until the early 1900s that they became popularized for Christmas. My grandmother, who dabbled in them briefly, remembered when they started becoming a big commercial thing in the 1920's. Also, be aware of climate. Don't have big mounds of geraniums or big hydrangea bushes in places with the climate of Nebraska or Scotland. They won't grow, okay?
For that matter, research climate if you're setting something in a place you haven't been. I remember being 13 and writing a story where England had the same climate as California. By the time I was 18, I winced every time I thought of it. But I've seen published works make the same mistakes.
If you're going to talk about money in detail, for godssake do some research on what things cost! I'm currently reading a historical mystery where I have come close to stopping reading about every 10 pages, because the monitary system is just so wrong. It's 1837 England, and Our Heroine is tipping maids 6 shillings, and paying a guinea a night for lower-class lodgings. No wonder she's worried about going broke! Our theory is that the author found out what the conversion rate was about the time England went to decimal, in the early 1970s, and used that. Let's see, I know I paid 16 pounds a night for bargain lodgings in London in 1975, and a lot less in the country. Conversion rate about $2.35 to the pound sterling...so $25-30 a night. Yes, we're in the 1970's. Nice try, but no. If memory serves, the British pound was worth about 8-10 times what it was in the '70's. I know you could buy a loaf of bread for a penny, and a book or a cheap hat for a schilling. I also seem to recall that someone worked out that Bob Cratchitt was making something like 220 pounds a year. In America, a lower-class working man made around $1 a day. I'm sure he'd love to be tipped by Our Heroine. Bottom line: if you don't know what things cost, be vague.
Be careful about food. I've read too many stories with people eating fruit in the dead of winter. And how 'bout those zucchini in March?
One nice thing about an alternate 19th century that has werewolves or steampunk is you don't have to be credible to the time period. It's already in another world.