A big thanks to Jim Bumgardner (krazydad.com) for motivating this Cartoon about Slitherlink puzzles. I had not played this before, but it is quite absorbing. His site provides many sets (books) of 16 Slitherlink puzzles in PDF format, along with their unique solution. They are organized according to level of difficulty. In every even numbered book set, the number counts in each puzzle are symmetrically placed, but the solution is definitely always asymmetrical.
In his instructions, he suggests that dot connections that cannot logically occur should be indicated by a thin x so that the remaining possibilities stand out. In the puzzle above, there would be an x between the 3 and the 0, enabling the 3 to be fenced as shown.
Slitherlink is also known as Takegaki along with other names in Wikipedia. The site also provides solution strategies. Another site that has interesting tutorial is the Nikoli puzzle site. Slow motion animated practice always trumps verbal description.
puzzle-loop.com has an FAQ that is very helpful in visualizing what can and cannot be drawn. Puzzles there range from sizes 5×5 to 25×30 in difficulty levels Normal and Hard.
Hirofumi Fujiwara has provided an excellent (non-obvious) “Key to Solution” set of basic, general and strategic rule sets. His site Puzzles And Java World also has puzzles online written in Java including Sudoku, Paint by Numbers, Cross Sums, and Sliding Piece Puzzles.
So don’t sit on the fence! Extend it!