Distinction between "ni"and "de"

I read from some book that there is a distinction between the use of に and で.

The book mentioned that で is used when the places or locations can be reached by people

ie. 私は時々レストランで食事する。
　彼女は日系企業で通訳をします。

and に is used when the places or locations can not be physically reached by people.

ie. 黒板に字を書きます。
　柱に釘を打つ。
　コップにお茶を入れます。

but...

What about this case? The book doesn't mention why there is an exception here...

父は今、庭に花を植えている。
父は今、庭で植木鉢に花を植えている。

Both are possible but the meaning is slightly different...

父は今、庭に花を植えている。

Here に indicates where the flower is being planted.

父は今、庭で植木鉢に花を植えている。

に is exactly the same as the another example, now the flower is being planted onto the pot instead of (the ground of) the garden.
で here indicates where the activity is taken place.
(Activity = father planting the flower)

If you say
父は今、庭で花を植えている。
he could be either planting the flower directly to the ground, or putting it in a pot.

The book mentioned that で is used when the places or locations can be reached by people and に is used when the places or locations can not be physically reached by people.

This is the first time I hear of such an explanation :)

"...de (place name)" refers to the location where, as explained by the earlier poster, where the "activity takes place."

私は時々レストランで食事する。= I eat AT/IN restaurants at times.
彼女は日系企業で通訳をします。= She interprets AT a Japanese corporation.

...both marks the place where "eating" or "interpreting" is done.

"...ni" has an essential "sense of direction" *towards* something or someone.

黒板に字を書きます。= I write ONTO the blackboard.
柱に釘を打つ。= I hammer nails INTO the column/pillar.
コップにお茶を入れます。= I pour tea INTO a cup.

To give you examples using "ni" and "de" with the same place-name::

私は毎朝学校に行く。　= I go TO school every morning. (see the sense of direction)

私は毎日学校で勉強する。= I study every day AT school. ("studying" takes place AT/IN school.)

Thanks AK and the one that explain the differences between the two particles.

It is much more clear now.

However,

I have another question, isn't へ　also used for direction? Any differences between this an the other two particles?...