The most popular books released in the current month is reached by clicking on popular in the drop down explore menu (click on the small arrow beside the word explore in the Goodreads header).
I wrote an earlier post about the Goodreads new releases email If you follow the more new releases link from the email, it takes you to the most popular books released this year, but if you go to the new releases page first and then click on more new releases, it takes you to a list of the most popular books released this month. These lists by date are only updated once a week.
Once on the most popular book page, you can select the year or month and year. Click on the view button after changing the selection. You can also switch between month and year lists. It currently only allows you to select months through December 2012 and years from 1911 to 2011. A workaround to go farther into the future is to change the year or month and year in the URL.
Under browse books are several other links. Discussion links to the book talk page. I explained Listopia in a previous post. Most read leads to a list of books marked read this week in your country, but the selections can be changed to this month or this year and the world. Popular books by date goes back to the most popular book page for the current year. Popular bookclub books is a list of books read by Goodreads groups that has the same options as Most read. Popular shelves links to the Top Shelves page. Recent reviews goes to Recent Reviews of all Books and it has a link to recent reviews of my books.
You can also find new releases from many genre pages.