I can highly recommend Tying Nymphs by Randall Kaufmann. It is laid out with easy at the beginning progressing to more complected later in the book. It is in its 3rd edition so that tells you how popular it is. The pictures are excellent. If you work your way through this book tying all of the flies you will know how to tie nymphs for sure. Including Randall's great stone nymphs. I also recommend you get the Spiral Hardcover. It lays flat and is worth the extra money.
This book is not cheap but I always buy books as investments. You will also get some history lessons about early fly tying in California.
Tying Nymphs by Randall Kaufmann