For modeling and other basic things and even advance, those tutorials inside the Autodesk Inventor are good enough. And for specific topics as you mentioned constraints, those might not be good enough.
You better learn from some teacher for specific knowledge or find some book. One book which I can name is for Autodesk Inventor 10 and is "Engineering Design and Graphics with Inventor 10 written by James D. Bethunes"
Always a good idea to check out the Autodesk AREA website. I'm not sure that they cover Inventor, but it's an excellent resource for learning the main modelling and animation packages they provide (i.e. 3DS Max, Maya, Mudbox etc)
First start with creating basic models, say a classic wooden chair.
However, if you really want to master inventor you have to know the software philosophy which is based on the SKETCH. Everything in inventor starts with a sketch. A good constrain for the design in inventor is done in the sketch. The fewer sketches you have in a single part the better the control you have over your design. Remember also sharing a sketch to create more than one feature from the same sketch is very powerful, but you would need more time thinking before you create this first sketch.
Say hello to youtube. Video tutorials should start to be your best friend. I resorted to youtube countless times to learn tricks. Some of which are not even meant to be created iin inventor, but masters of design do exist. Lucky for us many of them share there knowledge online.
Moreover, try to get an additional course in programming within inventor, after you have mastered the basics.