It depends a little bit on what you mean. The LIB keyword is to reference to a datasource by name, that has been created by you (or shared to you?), like this:
LIB CONNECT TO 'MyDataSource';
SQL SELECT * FROM `Sales_data`;
The other use of lib is when referring to a library as a folder (or similar), like this:
LOAD * FROM 'lib://PublicData' (html, table is @1);
The thing here is that in Qlik Sense Desktop, I don't think you can share these LIBs (like the first one) between applications. They are application specific. You could expect that this is something to look out for in the continuation of Qlik Sense later this year, without saying to much.
More information: http://help.qlik.com/sense/en-US/online/index.html#../Subsystems/Qlik_Sense_Desktop_help/Content/LoadData/ConnectToDataSources.htm%3FTocPath%3DWorking%2520with%2520Qlik%2520Sense%7CLoading%2520data%7CUsing%2520the%2520data%2520load%2520editor%7CConnect%2520to%2520data%2520sources%7C_____0