- If you're interested in learning a new opening, I recommend considering the following:
- Know your preferred style of play: positional or tactical
- Know your areas of strength: memorization, calculation, tactical vision, stamina, cool-headedness, etc.
- Decide how much time you want to spend learning (and keeping up with) the opening
These factors will all influence the decision of which opening to study; don't do this by feel. For example, successfully using the London System is largely based on strategic concepts; there are very few traps and forcing lines. As a result, the established theory isn't very deep. The King's Indian Defense, on the other hand, is the complete opposite: it's full of traps and forcing lines, and the theory runs to 20 moves and more in some lines. Just look at Beating the King's Indian and Benoni, by Anatoli Vaisser, which is almost completely dedicated to the 4 Pawns Attack variation, and has an average of 7 games per page for 114 pages (that's nearly 800 games) as reference material.
Unfortunately, we can't assume that because a few of the same moves occur in two openings that they are similar in nature, either in the degree of complexity or the strategic / tactical balance. The "...e6 Sicilian" you refer to includes the Taimanov, Szen, Pin, Kan and Closed variations, as well as the Marshall. The French Defence has almost nothing in common with these, largely because in almost every French variation White plays either e5 at some point or exd5, while in the 195 distinct lines of the Sicilian Defense (by ECO code), White plays e5 in only 7 of them, and exd5 in only 1.
This makes the French a cramped, counterattacking defense where White holds d4 and even e5 as long as possible, while Black tries to liquidate the e5 pawn and open the f-file. By contrast, in the Sicilian White loses the d4 pawn immediately in all of the Open-related variations (Closed and Alapin are the main exceptions), and Black fights on the half-open c-file. Not much in common there.
However, there are some similarities between Black-and-White mirrors of the same opening, such as the KID and the King's Indian Attack, and the Sicilian Dragon and the English Sicilian Reversed or the English Bremen, Reversed Dragon. Similarly, the Dutch Defense and Bird's Opening have strong similarities.
Cause of poor opening results: First, you have to determine that the opening was the problem. An analysis of the games should determine that:
You played the first out-of-book move, when book moves with good results were available in the position, and
An engine determines that the positional evaluation dropped by at least 0.30 pawns as a result of the novelty
If these are true, then you don't know the opening well enough. If, on the other hand, the evaluation didn't change much after you played your last book move, then the opening was fine (and not the problem).
On value of books: Read the reviews. I find these extremely valuable. I was once interested in booking up on the Glek Variation of the KID as White, to prepare for an opponent. Several books I was considering claimed to address it, but the reviews revealed that the treatment was cursory, and I was able to eliminate them from consideration. But you need to be able to learn the opening from the book when you find it; read, practice, test, repeat.
You can certainly learn an opening by playing master games in it. Pick some where the players are equally rated, and a few where the stronger player was playing your opening vs a weaker player. Start with the imbalanced games first. If you can determine that the positional evaluation shifted during the opening (in the first 12-15 moves), you're looking at a demonstration of how to use the opening to win. Then, play through the games between the equally-matched players. Because they foil each other's intentions, you won't see the themes unfold, but you should be able to spot how a strong player avoided the problems a weaker player ran into.
I don't recommend this approach over using an opening reference, though. If the latter's any good at all, you'll save hours of time distilling the key elements and themes of the opening. For learning a new opening, I can strongly recommend the "Starting Out" series by Everyman Publishing; they publish in both paperback and e-book versions. Go from there.
- This is very much a "your-mileage-may-vary" situation. You can learn to play the London passably in a weekend (say, 4-6 hours), but you won't be decent at the King's Indian for months (i.e. you'll have a decent chance to win if you get to carry out the general ideas, but good opposition will very likely kill you with tactics and traps repeatedly until you're familiar with them and how to avoid them). That said, the KID leads to fewer draws and more decisive games than the London System, and if you're a good tactician and weak on strategy, the KID's a pretty good weapon.