The Book Marketing Show Podcast with Dave Chessen has a ton of valuable content. Some episodes to note:
Chessen is the creator of the book marketing platform, Kindlepreneur. which is chockful of fantastic free resources and guides to all things book publishing and marketing. Some articles to note:
Goodreads Giveaways: for $119, you can "giveaway" up to 100 copies of your e-book or print book (note, you have to ship the latter) to those who request a copy. Goodreads then reminds the winners of the giveaway to review the book
A rather sobering podcast on just how difficult it is to sell books these days, and why you need to hire a marketer to get traction. From the Write-Minded Podcast: Tough-Love Marketing with M.J. Rose
M.J. Rose founded AuthorBuzz. From their website: AuthorBuzz is a marketing service that puts authors directly in touch with readers, reading groups, booksellers and librarians. Since 2005 we have provided the foremost opportunity for authors to make personal connections and offer extra perks to the people who buy, sell, read and recommend their books. See their marketing packages here.
Here are a few other sites of interest:

The Complete Guide to Creating an Author Website by John Burke
45+ Author Websites With Stellar Designs by Diana Urban
BookBub's 119 Book Marketing Ideas to Help Authors Create Sales by Diana Urban
Amazon Ads--I've had some luck selling books through Amazon Ads. It's a pretty cost-effective way to get your book seen--you set the budget, and you only get charged if someone clicks on your book. The learning curve can be pretty steep and the set-up is ridiculously over-complicated. If you don't want to do it yourself, you can hire it out. One very inexpensive option is to hire a freelancer through the website Fiverr. I use their services often and highly recommend them! Just be sure to go with someone with lots of clients and high star ratings.
This site is an ongoing work-in-progress. Check back for updates!