[nrapallo]

Do you realize that blockquote is a reserved HTML tag name i.e. <blockquote> ... </blockquote>? It indents both left and right margins at the same time. Perhaps try that instead.
(see the second and third images attached which use blockquotes for each paragraph and nested blockquotes, respectively.)

I attach a .imp ebook made using just blockquotes as per this HTML code:

<STYLE TYPE="text/css">
td.coverpage {width:100%; height:445px; text-align:center; vertical-align:middle}
h1.booktitle {margin-top:15%}
h3.author {margin-top:5%}
h3.book {margin-top:7%}
p.blockquote {margin-top:1.5ex; margin-bottom:1.5ex; margin-left:3.5em; margin-right:3.5em}
p.blockquote_begin {margin-top:1.5ex; margin-left:3.5em; margin-right:3.5em}
p.blockquote_mid {margin-top:0.5ex; margin-left:3.5em; margin-right:3.5em}
p.blockquote_end {margin-top:0.5ex; margin-bottom:1.5ex; margin-left:3.5em; margin-right:3.5em}
.pagebreak {page-break-before:always}
header {display:none; display:oeb-page-head}
span.header {font-size:x-small}
span.footer {font-size:x-small}
footer {display:none; display:oeb-page-foot}

</style>
<br>
<blockquote>The Great Depression of 2020 fostered a revival of organized crime. The black market in technology surpassed drugs and prostitution to become the primary source of revenue for these criminals.</blockquote>
<blockquote><b>Dr. Jessica Owen-Wells</b>, The Great Depression of 2020, copyright 2041, American Historical Society </blockquote>
<blockquote>In creating the thinking machine, man has made the last step in submission to mechanization; and his final abdication before this product of his own ingenuity has given him a new object of worship: a cybernetic god.</blockquote>
<blockquote><b> Lewis Mumford</b>, The Transformations of Man, 1956 </blockquote>
<br>
<br class="pagebreak">
<blockquote>The Great Depression of 2020 fostered a revival of organized crime. The black market in technology surpassed drugs and prostitution to become the primary source of revenue for these criminals.<br>
<blockquote><b>Dr. Jessica Owen-Wells</b>, The Great Depression of 2020, copyright 2041, American Historical Society <br>
<blockquote>In creating the thinking machine, man has made the last step in submission to mechanization; and his final abdication before this product of his own ingenuity has given him a new object of worship: a cybernetic god.<br>
<blockquote><b> Lewis Mumford</b>, The Transformations of Man, 1956 </blockquote></blockquote></blockquote></blockquote>
<br>

Now addressing your problem:

Are you trying to reproduce that with your code using <p> only? If you want the text justified you might want to add the style "text-align: justify" to your <p> lines.

Your HTML code works for me as per the attached first image.

Perhaps, you need to use the most recent version of eBook Publisher (v2.3.15) available for download here.

What exactly would you like to see your HTML code do?