I'm always on the look out for great new history books, but searching on the internet for good information is quite difficult. There's lots of information on historical fiction, but not much for non-fiction popular history books, and when you do come up with something there's invariably a lot of historical fiction mixed in. Here's a few websites I've come up with that you might find useful.
http://www.historyextra.com/books - This is the books section of BBC History Magazine. There are hundreds of reviews from books reviewed in their magazine over the last few years, and you can browse and search by category or keyword. They're also starting book lists by asking leading historians to suggest five books on a particular topic.
Reddit History Books List - On Reddit (a social bookmark & link sharing site), the 'Ask Historians' subreddit community came up with a list of the best history books, by category and era. There's a lot of great books on there, and as well as the list you can see the discussion that is behind it. I even contributed a few books to that list. Obviously this is only a very small selection of the great books in each category available, but its useful to look at for inspiration as to what to read next or what you've missed before.
http://www.lovereading.co.uk/genre/his/History.html - LoveReading is a lovely site with hundreds of book reviews, book extracts/first chapters and more. In each genre (which includes non-fic history) they choose a few books each month to review and provide extracts for, which as it has been going for years means quite a big back catalog!
Goodreads History Book Club - This is a very active forum and book group (probably the most active history book forum online) that reads all sorts of history books - mostly non-fiction. They have several books the group reads each month and then discusses, as well as a variety of challenges. If you want to though, you can just browse the forums and join in whenever you like.
History Books Review Blog - I'm always on the look out for good blogs, and I've recently come across that one which is really good. This guy's been posting regularly for about 5 years which is a very good sign in my book!
I'll be updating this as and when I find other good web resources, and if you know of any please let me know and I'll add it.