Signese

1) Not the first one, but the second one is

Please adhere to the rules of borrowing books from the public library.

Please don't bend the pages of the books.

Please don't write in the books.

Please maintain the books' purity (cleanliness, etc.).

Thank you for your cooperation.

Library Administrator

or something to that effect, anyway.

2) There were two characters whose order seemed to consistently be reversed, but that's all I noticed.

I'll leave the other two alone.

印張19.25 - it seems to be some sort of printing/paper standard, something to do with 19.25 sheets of A1 paper ~ 308 sheets of A4 paper, or something relevant? Or something to do with the ink usage of printing such quantity on both-sides of the paper in a single-colour/單色 (black and white)?

I don't know.

<p>I think 印张 means the number of A0 paper sheets used to print one book. I think.</p>

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3) I like the handwriting, it's all nice and 草, though I can't make out more than

馬年？月於北京 (and I'm only guessing at the 馬)

2)

First sign:

I prefer 请不要在书上写字.

Maybe 原则 would be better replaced by 规则 or 规矩.

图书管理员 or 图书馆管理员?

把书本折起来 is slightly odd, but maybe it's because I imagine most books are rather hard to fold. Maybe 把书页折起来 instead?

2nd sign (in simplified):

Seems okay to me.

@Glenn: I think that those two characters when in "reverse order" actually make up a word.

@creamyhorror: thank you for your effort on the scrawled message. I'd really like to know what it says And the first grammar issue you mentioned is precisely what I found suspicious, glad to see I was not mistaken.

@roddy/skylee/xiaocai: the book has about 300 pages i.e. 150 sheets of 26*18cm not sure how it computes.

(And while checking this I noticed that in addition to the handwritting it also has tears and dog-eared pages )

So most likely the book is in 十六开本 (A4), which basically means that 16 sheets of this book was made from 1 全开 (A0) sheet, and I think you can complete the rest of the calculation from here. And bare in mind, according to my friend who is a librarian, when referring to 印张, for double sided print, both sides count. 百度百科 seems to agree with what she said.