ruurubukku ha mizikai ni kosita koto ha nai.

No. In normal Japanese, it would usually be written as:

ルールブックは短いに越したことはない。

The spaces here are just a learning aid to make the parts easier to see.

に越したことはない is a set expression meaning:

• there’s nothing better than ...
• it’s best if ...
• ... is preferable

So ルールブックは短いに越したことはない means something like When it comes to rulebooks, shorter is better or A rulebook is best when it’s short.

Because に越したことはない attaches to the plain form of an い-adjective, and 短い is that plain form.

• plain form: 短い
• adverbial form: 短く

This pattern does not take the adverbial 短く here. So 短いに越したことはない is the correct form.

Here, 越した is read こした.

So the key phrase is read:

にこしたことはない

This is a very common fixed expression, and learners often memorize it as one chunk.

越した comes from the verb 越す, which can mean to go beyond, to exceed, or to surpass.

So, very literally, に越したことはない has the idea of:

there is nothing that surpasses ...

That is why it ends up meaning nothing is better than ...

Even though 越した looks like a past form, this expression is a fixed idiom. It does not mean past time here.

They have different roles.

1. ルールブックは

   • This は marks ルールブック as the topic: as for rulebooks

2. ことはない

   • This は is part of the fixed expression に越したことはない
   • You should think of it as part of the grammar pattern, not as a separate topic marker in the same way as the first one

So the sentence is not strange just because it has two は.

こと nominalizes what comes before it, turning it into something like the matter of being short or the case of being short.

In practice, though, it is best to learn に越したことはない as a whole expression rather than trying to translate こと by itself every time.

A rough literal breakdown is:

• 短い = short
• に越した = surpassing that
• ことはない = there is no such thing

So the overall idea is nothing surpasses being short.

It could be either. Japanese nouns usually do not show number by themselves.

So ルールブックは could mean:

• as for a rulebook
• as for rulebooks

In this sentence, it most naturally sounds generic, meaning rulebooks in general.

They are similar, but not exactly the same.

• 短いに越したことはない

  • short is definitely preferable
  • sounds like a judgment or piece of advice
  • often has a practical tone

• 短ければ短いほどいい

  • the shorter, the better
  • emphasizes a scale or degree more directly

So both work for similar situations, but に越したことはない is a more idiomatic nothing beats X type of expression.

It is fairly natural and common, but it sounds a bit more polished or idiomatic than very basic speech.

For example:

• ルールブックは短いほうがいい。

  • more straightforward
  • A rulebook is better if it’s short

• ルールブックは短いに越したことはない。

  • a little more idiomatic
  • Nothing beats a short rulebook

So it is not stiff or unnatural, but it is slightly more formal or literary than the simplest alternative.

Yes, very often.

For example:

• 行かないに越したことはない。

  • It’s best not to go.

• 無駄遣いしないに越したことはない。

  • It’s best not to waste money.

So the pattern can be used both with positive things that are preferable and with negative actions that are better avoided.