Games Workshop has started to release smaller supplements to the 40k and fantasy universes. At first look it seems that GW is nickel and diming their customers once again and for the most part yes but it isn't as bad as one might think. The new content digitally is much cheaper than the physical book, this is not always the case but GW is doing this with some of the new content.
What is the new content? Well, they are giving codex supplements these for the most part have been faction addition to any codex release such as the Black Legion for Chaos Space Marines. These are kind of frustrating release by GW, because one would think that these supplements should come with the codex. Some of the other new content is the dataslate, these so far are basically a small codex. I picked up the digital copy of Be'lakor, and I was happy with the price and the content. The e-book was basically some fluff and all the rules needed to field the unit.
All in all the idea of the new rules and fluff to be released more often than not is better for all factions. This idea to release new unit in a dataslate gives Games Workshop the ability to help rebalance some armies when their codex is getting stale and for the price you can't go wrong. Going forward expect to see more of these small supplements, I would venture to say that new models will have a dataslate to accompany them prior to a release.