Page#1
Goto URL – http://edelivery.oracle.com/
Click Continue button
Page#2
Give your name, company name, valid email id and country. Click Submit below.

Page#3
Select “Siebel CRM” from drop down and platform as “Microsoft Windows(32-bit)”. Click Continue.
Page#4
Search for “Siebel Business Applications Release 8.1.1.0 Media Pack for Microsoft Windows (32-bit)” on this page. Click that link.
Page#5
Look for Seconf Download button on the page. Or search “V14591-01″.
Here is your Siebel Bookshelf 8.1.1.0 Version.

Here is a link that Oracle Provides to get the avaliable Siebel Licence Keys –
http://licensecodes.oracle.com/siebel_master.html#SIA81
How to Apply Licence Keys in Siebel – Click Here.

Download Oracle and Siebel Setups:
Here is a link that Oracle provides to freely download the Siebel Setup for versions Siebel 7.5, Siebel 7.8, Siebel 8.0, Siebel 8.1.
You need to register first, what ever free Siebel downloads available , you can do there from Oracle Site.
http://edelivery.oracle.com/
Thanks, Ramani K for providing such a useful link.