If you're 1335, it's likely that you are still weak tactically
(dropping pieces to two move combinations) and don't know how to handle
basic endgames.

To improve your tactics--which is 90%, at least, of chess on this
level--there are many tactics manuals that teach you fundamentals like
line clearance, interference, basic mating patterns, etc. Virtually any
one of them would do at your level. You might also consider getting
tactics training program for your computer.

To improve your endgame, try Fine's "Basic Chess Endgames", a true
classic. Concentrate on those endings you actually face--i.e., pawn
endings, rook endings, and minor pieces endings (With pawns). Only
rarely on this level do you reach a queen ending, or a complicated
pawnless ending, for example.

Openings I wouldn't worry about too much in your games. Dirty little
secret: for most class players, studying openings is almost a complete
waste of time. Think about it: suppose you know the Caro-Kann main line
up to, say, move 15. What are the chances that your opponent does? If
he deviates from the "book" at move 4, do you know why the move he made
is inferior? Almost certainly not.