Assuming you want to learn for PICmicros, google
"PIC Googlium tutorials"
I suggest you get an enhance midrange PIC and start there.
I actually learned through a mishmash of online tutorials (90% of which are primitive and will give you bad habits) and the datasheet. And it took me like 6 months to even blink an LED in assembly. I ended up going to PICaxe, first, just trying to learn the language (not assembly, just the language of micros and circuits in general, enough to decipher and put the datasheet into perspective) and build some confidence, then coming back at the PIC.
When I finally found these tutorials, 90% of it was old hat to me by then, but the other 10% was solid gold. If I had this when I started, I would have had a 6 month head start. I had often thought of writing my own tutorial, but finding these killed that idea. This covers almost everything I know, and they are extremely well organized, concise, and well written. Whoever wrote these, he/she understands very well what he's doing. He understands very well what you don't. And how to teach. I couldn't explain anything any better than what is in these tutorials. (These tutorials are also kinda redundant. The more advanced ones include most all the basic stuff, so if you want to get the most bang for the buck, just start with the most advanced enhanced midrange PIC... go back to the most basic one if you have trouble.)
They cost a few bucks, but worth 10x that. There are famous authors with well known books. I have purchased one the most highly rated book from one of the the most famous author. And it barely skims assembly beyond author stating he knows how to write it... and here are some of my C projects. No real instruction and no practical help. These Googlium tutorials, OTOH, it's like a crime that they are so cheap. The knowledge that will be poured so easily into your brain. It's highway robbery. I ended up buying them all just to make sure I didn't miss any tricks... After I had been programming PICs in assembly for many years.
If you are new to electronics, in general, the free-to-download PICAXE manual has a great primer. One of the sections goes over basic electronics specifically pertinent to controlling things with a microcontroller.
Edit: Correction. I just remembered how intimidating and incomprehensible is a 300 page datasheet. If you're new to micros, I would suggest you actually DO start with the basic device. You WILL want to print out the entire datasheet in hard copy. You will be referencing it, nonstop. The basic devices will have datasheet 1/4 the size.