for useful resources for writers in various genres all I could think of was some useful sorts of books. I will leave internet resources up to other people. There are plenty out there. They change all the time. They come and go - like blogs - and often do not get updated. I suspect people feel great enthusiasm for a project in the beginning and then they get waylaid by other things or the work in keeping a site updated just proves to be too much.
Books are a different matter. I own rather a lot of books and rather a lot of them are reference books. I use them in my day job. A lot of them are dictionaries but there are other useful books as well.
With Nicola in mind I surveyed my shelves for books that might be useful to writers. I am not sure that these help with any particular genre but I find these books useful and interesting. I have a world atlas. Think your geography is good? Spend some time with an atlas. I also have road maps of Australia, the UK and Europe and a number of other 'useful' maps of more remote places. I have never travelled far, certainly not as far as I would like, but I need to know where things actually are, not just where I think they are.
I have books of names and place names. The books of names are particularly useful. It is good to know whether I am dealing with a male or a female and sometimes nice to know what their names mean. If you are writing a book that can be essential. It is also useful to know that names like "Wendy" are a modern creation (JM Barrie) and not something you can give to a girl in Roman Britain.
I have a book about medicinal herbs - often grown and used when no other medicine is available and, of course, used before the pharmaceutical companies took over. If I wanted to write historical fiction I am sure that would help if my characters were ill.
I have a book about islands and another about French manor houses, a book about the development of modern medicine, books about psychology, statistics, Celtic mythology, Irish literature. There are books about Gaelic verbs, Hebrew, Swahili, Maori, Urdu, Chinese and other assorted languages. There are books about music, art, religion, philosophy and linguistics. There are books about nursery rhymes as well as of nursery rhymes. Reference books about children's literature, reading and literature are also on the shelves. All of them are potentially useful for writers and I have used all of them at one time or another.
Many of them are books I have rescued from library book sales. It would be better for the authors if I could buy the books new but at least this way they do not make landfill.
I may have too many books. I know there are a number of people who think that. They claim they are "not readers". One of them claims not to have read a book since he left school. He is now 70.
I could not live that way. If you are a writer you must also be a reader.