Since it has the leading zero--which is good usage, because sometimes the decimal point can fail to print well--I would read it as "oh point two five." Just like "nought point two five" but using "oh" instead of "nought." Over a telephone or in any other situation where I was being very careful, I would say "zero" instead of "oh," "zero point two five." Since there are only two digits, it would also be common and natural to read it as "oh point twenty five" or "point twenty five."

Point two five.

Also, in American usage, commas are used to separate groups of three digits before the decimal point. The decimal marker is a period. After the decimal, no punctuation is used, although sometimes spaces may be used to group the digits into threes. Before the decimal, commas are not read aloud; instead, the words "thousand, million, billion" etc. are used, and American custom is to use the "short scale" in a billion is (only) a thousand millions, and so forth. Thus: 1,234,567,890.25 is "one billion, two hundred thirty-four million, five hundred sixty-seven thousand, eight hundred ninety, point two five." I've heard people talk of "the two-comma club," referring to someone who has a million dollars or more and thus needs two commas to write down the amount of money he has.

Americans do not use a comma to indicate decimal fractions. 0,25 does not make sense in U.S. usage. The book must contain a printing error. Perhaps the author wrote 0.25 and a careless copy editor--or a computer grammar checker--"corrected" it.

I don't know. I am not familiar with AE. The book talked about some numbers of the US and gave suddenly "0,25". I was totally confused, that is why I asked here. I do not think that AE is so different that they used something instead of "nought POINT two five"...Maybe it is a printing error in the book.