Last week I completed book Internet Multimedia Communications Using SIP from Rogelio Martinez Perea. This book is definitely recommended for beginners trying to learn SIP, even veterans can clarify their doubts using this book. The book points to all the RFC and JSR's where ever applicable and hence acts as good reference. Book refers to Mobicents as open source implementation of JAIN SLEE and NIST SIP stack as implementation of JAIN SIP. In fact all the examples are built using the JAIN SIP stack and Java Media Framework (JMF) for media. This shows how Open Source is making an impact in VoIP world :)
The part that I felt could be improved is the explanation of SIP Transaction Sub Layer.