Are there any reliable online resources for writers of historical books?

Thank you everyone, for all the great information and suggestions.

Consider following reference links at the end of Wikipaedia pages. They often lead to academic libraries who have put content online. As an example, look at where this article on William of Malmesbury leads:

en.wikipedia.org/wiki/William_of_Malmesbury

I wonder about local history societies. My local one has a wealth of information

The British Library has lots of things online.

www.bl.uk/

There is

Treasures in full

www.bl.uk/treasures/treasuresinfull.html

which includes Shakespeare quartos

www.bl.uk/treasures/shakespeare/homepage.html

and

Festival Books.

www.bl.uk/treasures/festivalbooks/homepage.html

As Geekesse suggests, I would phone or email a few academics in your field/era for pointers re which material to start with - then one source will lead to another. Usually academics are quite pleased to be asked, as long as you don't ask for help more than once!

Artemis 1, I have written a couple of published history books using original sources, eg original letters, photos, newspapers from the time, church records etc.
Whilst I enjoyed pottering around in university and national libraries and various archives, a lot of the info had already been digitalized and available on line.
Where I live, you need to contact the libraries and register the nature of your interest to access their archives. It may be the same where you live.

Doodledog's advice is good too.
Hope it goes well.

You might try tracking down a historian who specialises in your period, and try asking if they would identify some useful journal articles etc for you. You could offer a modest fee for the service, but it would save you a lot of time because you wouldn’t have to trawl through all the dubious sources on the internet.

Is there a society for the period you want to cover?

If so, they often have links to resources that will be reliable. The Richard 111 Society, or the Anne Boleyn Society are examples that spring to mind, but there are others.

Another useful resource is Academia.com. After the first few uses (or maybe the first length of time) they charge a small fee, but they have countless articles that can be downloaded, all referenced and researched, so that would be another good place to look.

You can also search Google Scholar, but a lot of the links lead to articles or books held in academic libraries, and you would need an academic email address from an institution with a licence.