As expected, the adverbial form (aka 連用形) of an i-adjective `...く` can be used
as an intensifying adverb to an adjectival predicate:

> すごく速い

But there are cases where the indicative non-past affirmative form (or
attributive form, depending on the theoretical assumption) `...い` is used in
addition to or replacing the adverbial form:

  * Young people's colloquial speech:

> すごい速い

  * Kansai dialect:

> えらいきつい性格

  * Modern literature:

> 恐ろしい沢山書いた (夏目漱石)  
>  恐ろしい長い物を捲り上げる (樋口一葉)  
>  恐しい利く唐辛子だ (泉鏡花)  
>  可恐い光るのね、金剛石 (尾崎紅葉)
>
> ([饮水思源 - 主题文章阅读:
> 形容詞の副詞化](http://bbs.sjtu.edu.cn/bbstcon,board,Japanese,reid,1181906808.html))

What is happening with this usage of i-adjectives?
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question
i-adjectives adverbs
Indicative form of an i-adjective used adverbially
This is an interesting topic but I think the question could stand to be a bit
more focused. I will throw out an answer in an attempt to inspire other people
to dig up better info and perhaps the OP to make the question more specific
and answerable.

So "what is happening", in general terms to avoid specific theoretical
assumptions:

> In certain kinds of speech, almost always non-formal, usually associated
> with young people, we see adverbial use of adjectives in their -i form (e.g.
> /osorosii/, /sugoi/) where Standard Japanese requires their -ku form (e.g.
> /osorosiku/, /sugoku/).

Points of interest:

  * The usage is often associated with young people but not necessarily a particular generation. However, particular _words_ are often associated with a particular generation. Your examples from "modern literature" (all 3+ generations old now) are all /osorosii/; the modern canonical example in Eastern Japan is /sugoi/. This suggests that _specific words_ used in this function might be dependent on generational slang, but it seems that the _pattern_ of using -i adjectives this way is stable over time (or is prone to repeated reinvention).

  * The usage seems to be restricted to adjectives which are "semantically bleached" and just convey intensification. I do not think you would be able to find "赤い光るのね" or similar.

  * The usage is not limited to one-word "natural adjectives". In natural speech you can hear things like "ありえない暑い" and "はんぱない暑い". It seems that anything that is adjective-like and has a general intensifying meaning can in theory be subject to this process. (By "adjective-like" I basically mean "has the -i ending", but I also mean to include things like ぱねー etc. that are _derived_ from the -i ending.) **EDIT:** You know what? I don't actually think that adjective-likeness is a requirement. See second-to-last paragraph below.

  * This is from memory, but I believe that in 若者言葉に耳をすませば, Yamaguchi Nakami argues that adverbial /sugoi/ is a _different lexical item_ than the adjective /sugoi/, citing as evidence the fact that speakers use _both_ adverbial /sugoi/ AND /sugoku/, but in different situations (and not just differentiated by formality etc. -- the meanings are slightly different).

  * It is interesting to note that we see something similar in informal English: compare "That guy is mad/crazy/wicked ugly" to ?"That guy is madly/crazily/wickedly ugly". 

Perhaps what is happening is that the semantic bleaching of the adjectives is
accompanied by a sort of "morphological bleaching", so that they are no longer
firmly in the "-i adjective" category any more for those speakers -- they
become general intensifiers with no particular inflection required that happen
to end in /i/.

**EDIT:** In fact, I'm less and less convinced that the original class of the
word has much to do with it at all. ちょう, めっちゃ etc. are examples of words that
seem to be the result of a similar process but were not originally -i
adjectives. Perhaps the -i adjectives just stand out because their non-
inflection stands out as an "error" when compared with the standard adverb-
forming -ku ending.

(We should note that these speakers may still keep the actual adjective in
their lexicon as well, a la Yamaguchi cited from memory above, so maybe it is
better understood as a form of branching.)
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answer