Without your help, writing this book would have been very difficult.
Straightforward sentence, perhaps by a person speaking from direct experience. The comma is important to me because without it I read "your help (in) writing this book" and have to back track when I get to "would".

Without your help(,) the writing of this book would have been very difficult.
More abstract sentence, taking the writing of the book as a notion, a thing, rather than an action, as if we saw the writing but perhaps were not involved in it.

Without your help, to write this book would have been very difficult.
This sentence takes us back to before the book was begun and suggests difficulty even in beginning to write the book.
https://forum.wordreference.com/threads/doing-something-vs-the-doing-of-something.1039965/

Также см. комментарий выше (там предположение об обычном употреблении предлога перед герундием).
Мне кажется, с глаголом звучит более естественно.
Тем не менее, часто можно найти примеры с герундием.
Я думаю, что когда используется герундий, более подчеркнут сам процесс.
Если что-то найдете на эту тему, пожалуйста, поделитесь, сам задавался этим вопросом, но до конца так и не разобрался.

Строгих правил нет, часто можно так и так. С герундием часто бывает "красивше" и короче.

Но это лишь средства выражения, а выражать предстоит вам.
Не забывайте только правильно обращаться и с существительным, и с герундием. Вот тут показаны отличия:
http://www.study.ru/lessons/upperint2-7.html

It depends on the nature of what is being done. In your sentence "designing clothes" (I presume you mean clothes and not "cloths") is an occupation in general rather than a specific or unique task that is being done at the moment. For this reason it would usually have the definite article "the", and with this we would use "of":
"The company is engaged in the designing of clothes."
Here's another example, a quotation from the New York Stock Exchange site:
"Texas Instruments Incorporated (TI) is engaged in the designing and making of semiconductors..."
These sentences describe what is being done by the subject generally, and in both cases is a profession that a number of people do. I'd say, though, that this kind of construction sounds formal or old-fashioned, and in fact rather awkward, I think, to modern ears.

However, if the subject is engaged in a particular or unique task, we would neither use "the" or "of":
"Mr. Dibdin is engaged in writing a list of the classical library of the Earl of Spencer."

To get back to the topic sentence, if someone is designing clothes for a particular purpose, we would omit "the" and "of", as in the last example above:
"The company is engaged in designing costumes for the new Star Trek movie."

As far as I can tell, using "of" in these last examples (that is, without "the") is either unnecessary of in fact wrong, and you must use "of" in the (general) cases where you use "the".

http://forum.wordreference.com/threads/doing-something-or-doing-of-something.2232505/