I haven’t done any of these for a while, so duck while I lob my backlog out there…
This is an interesting question, because the resources needed vary depending on where you are in your career path. The information you need when you first begin (what exactly makes a compelling protagonist?) is very different from what’s wanted after you publish (where can I advertise my books?). So here are some sites that have been useful to me as I developed my writing and publishing skills.
For writing: Mythic Scribes
When you’re in the early stages of writing – your first book, or perhaps still dabbling with world-building – what you really need is a community of like-minded people. Even when you’ve read all the craft books, it can still be tricky to apply the advice to your own work. Should I introduce my antagonist earlier? Is this a punchy opening paragraph? First person or third? To prologue or not? And fellow authors are the only people who can endlessly mull over those difficult questions of adverbs, passive voice, show-don’t-tell and so on without getting bored. And for fantasy writers in particular, there are not many places where you can ask how long it would take a person to die from a sword wound (although I imagine writers of murder mysteries and gun-based thrillers have pretty awkward research topics, too). Mythic Scribes is a forum for fantasy writers, and it was a huge help to me when I first started writing seriously.
For critique: Scribophile
There comes a point when you have something written that you’re quite pleased with. Finally, after all that struggle, something that might be publishable! But first, it’s vital to put it in front of other writers to see what they think of it. Can’t you do that with a writing forum like Mythic Scribes? Of course, but to my mind it’s better to show your work in a place that’s geared specifically for critique, full of objective strangers who won’t tone it down because they chat with you about Game of Thrones in another part of the forum. Scribophile is my favourite critique site. You earn points (’karma’) by critiquing the work of others, then you spend karma to have your work critiqued in turn. Not all critiques are useful, but collectively they are acutely rigorous and analytical. And there are forums and special interest groups as well.
For testing the waters: Wattpad
The disadvantage of critique groups is that, because it’s focused on single-chapter analysis, it’s hard to get a perspective on how a whole book looks to a reader. You can try to find beta readers for this, but one alternative is Wattpad. This is, strictly speaking, a social media site, which revolves around authors posting whole books one chapter or scene at a time. Readers follow the story as it unfolds and will comment on their reactions as they read each part. For author/reader interaction, it’s unparallelled, but the potential for objective critique is limited. It’s also possible, if authors write as they post, for readers to influence the route a story takes. I used Wattpad to post my first fantasy novel, The Plains of Kallanash, and it was a fun way to find out whether readers will follow the whole story or lose interest part way through, but it’s no substitute for detailed critique or beta readers.
For marketing and post-publication: Kboards Writers’ Cafe
Once you reach the point of publication, the focus changes. You’re no longer quite so worried about passive voice and overuse of gerunds, but about covers, ebook formatting, the vagaries of print on demand and how to get reviews. For self-publishers, there’s a wealth of information out there, but the best of it, and the most up-to-date, is at the Writers’ Cafe, a sub-forum of Kboards. This is populated by people who are, in the main, focused on self-publishing as a career, so the talk is more about writing to market and promotional campaigns than about writing as an art form. This is the place to meet other self-publishers, both those who are just starting out and those who have several years of experience under their belt, those who sell a book or two a month and those who earn six figures a year.
Footnote: Authors Answer is the brainchild of blogger Jay Dee Archer, of I Read Encyclopedias For Fun. You can read the answers to this question by his eclectic bunch of authors here. More recently, Erica Dakin, of the Theft And Sorcery blog, has been answering the questions independently. You can read her answer to this question here.