Hi Isio,

You can find more details about QWeb reports here: https://www.odoo.com/documentation/8.0/reference/reports.html
I also made a blog post about this here: http://www.odoo.yenthevg.com/creating-custom-reports-odoo-8/
And I'll explain things in detail here too:

I honestly don't find the doc too good so I'll give you a quick sample which describes everything.
Every Qweb report has an XML record in an XML file which refers to an XML file under views. The structure is always the same for every single module. Open up the module where you want to create a report. In the main folder there is an xml file which is always named moduleName_report.xml (for example sale_report.xml). In this record are the references to the reports from that module. For example on the sale module:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <report 
            id="report_sale_order"
            string="Quotation / Order"
            model="sale.order" 
            report_type="qweb-pdf"
            file="sale.report_saleorder" 
            name="sale.report_saleorder" 
        />
  </data>
</openerp>

As you can see there is a file statement, named sale.report_saleorder. This means that this specific XML record will look in the module sale for the report 'report_saleorder' under the folder sale/views.
If you open up the folder views (in the module sale) you will see there is an XML file report_saleorder.xml.
This is your actually QWeb report where you can add all the code you want. This is the absolute minimum code you need for a report:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
<template id="report_saleorder_document">
    <t t-call="report.external_layout"> //If you don't want an header & footer you can remove this line.
      <div class="page">
       <h3>Your title</h3>
      </div>
    </t>
</template>

<template id="report_saleorder">
    <t t-call="report.html_container">
        <t t-foreach="doc_ids" t-as="doc_id">
            <t t-raw="translate_doc(doc_id, doc_model, 'partner_id.lang', 'sale.report_saleorder_document')"/>
        </t>
    </t>
</template>
</data>
</openerp>

As you're using a custom module.. You can access any item (record) from your object 'book' by doing an o.fieldName.

The file statement from moduleName_report.xml (file="sale.report_saleorder") should have the same name as the second template id in 'report_saleorder'. If these don't match you'll get an error. For example with the sale report:

report_saleorder.xml:

<template id="report_saleorder">

moduleName_report.xml:

file="sale.report_saleorder" 

the template_id="report_saleorder" also has a t-raw statement:

<template id="report_saleorder">
    <t t-call="report.html_container">
        <t t-foreach="doc_ids" t-as="doc_id">
            <t t-raw="translate_doc(doc_id, doc_model, 'partner_id.lang', 'sale.report_saleorder_document')"/>
        </t>
    </t>

As you can see this t t-raw statement has 'sale.report_saleorder_document'. This is exactly the same name as the first template id in your report filename:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
<template id="report_saleorder_document">
//irrelevant code
</template>
</data>
</openerp>

These two also have to match or you'll get an order. If you've set this all up correct and update the module you will see the new XML report in your Odoo under the module where you specified it. 

NOTE: If you create a new, custom report you will have to add the xml file to your __openerp__.py declaraction or it will not be loaded / accessed. The filename of the new report under moduleName/views (for example demo_report) should then be added. Add this in the data statement. For example:

'data': [
//irrelevant code
'views/report_saleorder.xml',
'views/demo_report.xml',

I hope this helps you enough. Ask questions if you'd like to know more! And please accept/upvote if you like the answer :)