Literature

How-Tos, Documentation, etc.

General

ITP Physical Computing Tutorials: Several tutorials around PCB making and assembling
MAKE: How to make PCBs: Collection of resources and guides
Sparkfun Tutorials: Tutorials on electronics design including EAGLE and SMD soldering
OpenCircuits: Wiki for sharing open source electronics knowledge, schematics, board layouts, ports and parts libraries
MassMind: Wiki tech reference around learning to design and produce PCBs
Physical Computing: An introduction to Physical Computing by Dan O'Sullivan, Tom Igoe
Making Things Talk: Practical Methods for Connecting Physical Objects By Tom Igoe
Getting Started with Arduino: An introduction to the Arduino open-source electronics prototyping platform by Massimo Banzi
Fashioning Technology: A DIY Intro to Smart Crafting By Syuzi Pakhchyan

EDA terms: Terms used in Electronics Design Automation (which is essentially what Fritzing does) are explained at EDA Consortium, EETimes
PCB terms: Glossaries of terms used in designing and manufacturing Printed Circuit Boards can be found at Golden Gate Graphics, PCBpro, SunMan
Electronic symbols: Overviews of symbols for electronical parts used in schematic diagrams can be found at Electronics Club, Wikipedia (German)

PCB Design Tutorial: A popular quick guide on learning how to design PCBs, by David L. Jones
Build Your Own Printed Circuit Board: Effective book that covers the full process from design to production, based on EAGLE, by Al Williams