Drawing on the Right Side of the Brain is probably the best single book for complete novices, especially ones who did not draw in years. Otherwise you will need to resort to multiple works.
If you are still interested, go through Norling's book on perspective, and then move on to Loomis' books. If you are completely green, start with Loomis' Fun With a Pencil. Otherwise, you can skip it.
>Preferably something that's in the context of digital art with a tablet
Some people will disagree, but I think that starting out traditional is best. You are most likely familiar with pencil and paper, so you can focus on drawing rather than messing around with table's shortcuts and settings. Traditional methods also reinforce good habits, and prevent you from abusing crutches like control+z.
Translating skills from traditional to digital is much easier than other way around too. For most people anyway. It is something to consider, as if you take the art hobby seriously you will probably want to try some life figure drawing.
Others covered some things, but I think that the "flat" look and bad coloring are the biggest offenses.
Perspective is messed up. Guy's leg just sort of disappears into the cushion and we do not get an indication of any crevice that could be hiding it. Arms look awkward wrapped around girl's waist (we should see less forearm), middle finger's placement is off, and pinky is too long. All these tiny issues add up.
What you see on the second image is coloring's equivalent of symbol drawing. There is no consideration of where the light comes from, the way shadows are painted does not complement perspective, and blending is very basic. At the same time, some stock effects like the "blushed reflective" skin are used and they do not match the style of coloring. That's why light reflecting in sweat covered, rosy asscheeks looks like blisters as >>1313
Thinking about light source alone would fix a lot. There are candles in the back, so figures will be illuminated from the back. It means that parts closest to lights (outlines where characters are not in contact with the bed) will be the lightest. That's assuming there are no light sources off page, as they will complicate things. Still, surfaces closest to the light sources will be the brightest.
Besides that, the first image is sketchy and has no colors. It lets your brain fill the blanks subconsciously. It can't do that with the second image.
It's kind of like with horror movies where keeping monster somewhat hidden lets viewers' minds fill in the blanks, making it scarier to them. In case of pictures, your brain makes it look better.