Microsoft today released Windows 10 Mobile Insider Preview build 15043 to the Slow ring. While the Fast ring saw it only a week ago and it didn't bring a lot of new features, it's the first Slow ring build since mid-December, when 14986 was released for PCs as the last one before the holidays.
Naturally, you'll see more new features than the Fast ring did with this build, since it's been so long since the last one, which for phones, was 14965 in November. One of the big new features that you'll see is ebooks being sold in the Windows Store. These can be read through the Edge browser, and you can even have the browser read them to you.
There are some minor new features as well, such as a new Share icon, and the use of more colorful emojis in Edge. Of course, the emoji feature may be major, depending on who you ask.
Build 15043 is still the current one in the Fast ring for Mobile, since there wasn't a new one released this week alongside 15046 for PCs. Also, as it stands right now, app updates are on hold in the Fast ring, so if you want the latest features in apps, you'll need to be on the Slow ring anyway.