For text at undergrad level, google "first course loop" (w/o the quotes) and you get:
https://www.amazon.com/dp/0199590753/?tag=pfamazon01-20
For a more advanced treatment google "zakopane lectures loop" (no quotes) and you get:
http://arxiv.org/abs/1102.3660
Zakopane Lectures on Loop Gravity
Zakopane is a mountain resort near the Poland-Austria border where Loop Gravity schools lasting a week or two have often been held to introduce it for grad students and postdocs wanting to get into research. So this is one bunch of introductory lectures that has been condensed down into 30-some pages.
There are several more introductions and others may post links to some of them. There are highly regarded books by Thomas Thiemann and by Rovelli (who has a free online version at his website), also a paper by Hanno Sahlmann, a paper by Bilson-Thompson, a pedagogical review in the online Living Reviews collection, and a series of a dozen or so introductory video talks by Rovelli at PIRSA. If you google "pirsa rovelli" you will get, among other things,
http://pirsa.org/C12012 which gives links to the video talks.
Good resources, but not specifically introductory, are the sets of videos from the conferences Loops 2011 Madrid, and Loops 2013 Perimeter. The Loops conference is biennial, so the next one will be in 2015 and will be held at Erlangen, Germany.