They say the best way to improve up to a certain level is tactics.
I would like to stress the tactics point a bit further (farther? skip it as Groucho would say
It is very important to study tactics.
Ken Smith (owner of Chess Digest and master level player) used to say "If you are rated under 1800, your first name is tactics, your middle name is tactics, and your last name is tactics !!! "
The 1001 books by Reinfeld are great in this respect.
I think in addition to mere tactics with a side to win, it is useful to look at how those tactics came about (lack of development/grabbing material/castling to early/oversight/etc.)
Getting an idea of what caused the actual tactic to occur can help very much.
That's why I usually prefer miniature games over bare tactics.
1000 Best Short Games Of Chess by Chernev has 1000 miniatures where players of all levels go down in flames. Some games even have brief explanations of how the side lost.
Another good book is 100 Soviet Chess Miniatures by Clarke.
You kind of learn the ebb and flow of the game from studying the complete miniature.
Another good place would be to study gambit openings. Many beautiful tactics can be found in openings that were later proven unsound.
I am talking about lines like 1.e4 e5 2.Nf3 d5 3.Nxe5 dxe4 4.Bc4 Qg5 5.Nxf7 ? (5.Bxf7+ puts the line in trouble).
Here's some nice tactics from the Smith-Morra
1.e4 c5 2.d4 cxd4 3.c3 dxc3 4.Nxc3 Nc6 5.Nf3 d6 6.Bc4 Nf6 ?! runs right into e5.
1.e4 c5 2.d4 cxd4 3.c3 dxc3 4.Nxc3 Nc6 5.Nf3 e6 6.Bc4 Qc7 7.Qe2 Nf6 8.0-0 Ng4 when 9.h3 ?? runs right into Nd4 !!
There are obviously many more exciting variations from studying gambits.
I have actually been to that chess tactics site (emarald??) and didn't care for it. I felt rushed. The whole scoring system added pressure rather than enjoyment. I achieved 2100 in 1 minute chess (at FICS) just from studying miniatures, studying tactics in books, studying gambits, and experience from playing.
I hope this helps anyone out there.