Hi Kushal,
If you know v6.1 basically you will find more profile types, new ways to administer, fewer feature packs (previous are included but there are new ones), and in v8 new way to install and apply maintenance. In v8.5 there are more important changes.
These links will be useful:
System administration for WebSphere Application Server V7: Part 1 -- An overview of administrative enhancements
http://www.ibm.com/developerworks/websphere/techjournal/0811_apte/0811_apte.html
System administration for WebSphere Application Server V7: Part 3:
Administering a flexible management topology
http://www.ibm.com/developerworks/websphere/techjournal/0903_khalil/index.html
System administration for WebSphere Application Server V7:
Part 2: New administrative topologies
http://www.ibm.com/developerworks/websphere/techjournal/0901_cundiff/0901_cundiff.html
System administration for WebSphere Application Server V7: Part 4: Properties-based configuration
http://www.ibm.com/developerworks/websphere/techjournal/0904_chang/0904_chang.html
System administration for WebSphere Application Server V7:
Part 5: Business-level applications
http://www.ibm.com/developerworks/websphere/techjournal/0905_edwards/0905_edwards.html
Infocenter 7 New features
http://pic.dhe.ibm.com/infocenter/wasinfo/v7r0/topic/com.ibm.websphere.nd.doc/info/ae/ae/welc6topnew.html
What's new in WebSphere Application Server V8
www.ibm.com/developerworks/websphere/tec... Infocenter 8 New features
http://pic.dhe.ibm.com/infocenter/wasinfo/v8r0/topic/com.ibm.websphere.nd.multiplatform.doc/info/ae/ae
/welc6topnew.html
Redbooks:
WebSphere Application Server V8: Administration and Configuration
Guide (sg247971)
WebSphere Application Server V7.0 Security Guide (sg247660)
Hope this helps.
Regards