You will need to do some work in your CSS file. How much work depends on how well the epub was set up originally.
Look for something similar to:
p {text-align:justify}
and change it to:
p {text-align:left}
Hopefully, it is a clean document and you will only need to change it the one time...
BTW, what you want to do is exactly why it is beneficial to have a clean CSS sheet, with clean HTML tags. One minor change can fix the whole document!
FWIW here is an example of what I'm talking about
:
Spoiler:
There is (almost) never a need for using inline styles as you mentioned...using CSS keeps your document clean and easy to read. Use classes only when there is some special formatting that needs to be added to the standard formatting. eg. the basic <p> paragraph is left aligned with an indent of 1.2 character widths. If I want special formatting for a <p> then I would give it a class name like "first" or "ctr".
Later on if I wanted to change ALL the basic paragraphs to justified, then I only need to change it on one line and it doesn't effect any other paragraph.
Here is a quick example:
Code:
CSS:
h2 {text-indent:0; text-align:center; font-weight:bold}
p {text-indent:1.2em; text-align:left; font-size:1em; line-height:1em}
p.rt {text-indent:0; text-align:right}
p.ctr {text-indent:0; text-align:center}
p.brk {text-indent:0; margin-top:2em}
p.first {text-indent:0; clear:both}
p.first:first-letter {font-family:serif; font-size:2em; font-weight:bold;
float:left; margin:-.1em .1em -.1em .1em}
p.first:first-line {font-variant:small-caps; font-size:1.15em}
HTML:
<h2>Chapter 1</h2>
<p class="first">First paragraph in chapter...has large initial letter and the first line in the display is in small caps.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p class="brk">Paragraph with space before indicating a scene break...no indent.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p class="ctr">Paragraph aligned center.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p class="rt">Paragraph aligned right.</p>
<h2>Chapter 2</h2>
<p class="first">First paragraph in chapter...has large initial letter and the first line in the display is in small caps.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p class="brk">Paragraph with space before indicating a scene break...no indent.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
<p>Normal paragraph...standard formatting.</p>
caution: the :first-letter and :first-line selectors may not be supported by older, inferior, devices. They are only presented here as an example.