You're welcome
As for exercises, until you can get the book there's not much beyond studying from life and photography. It's really worth the investment to be honest, it sounds expensive at 40-60 dollars but the it's easily worth 10 times that, if not more, in terms of knowledge value.
But that's the boring answer! More specifically, I would recommend this:
Find a good grayscale photograph of a subject you like. You seem to like drawing insects and animals a lot, so maybe that could work, but the forms might be too complex and so might the perspective. You can try working from that and really focusing on the values. No colour, no switching between mediums. I think charcoal would likely be the best medium if you don't do digital yet. Pencil is good too, but the tendency I've noticed among beginners , and I did this too at first, is fearing to go too dark and pencil doesn't encourage boldness of value unless you really force it/work it in. But the subtlety required to properly show form with minimal value ranges is most likely out of your league at the moment. Generally, being subtle is much harder than being obvious. And if you want to learn about lighting, the best way is to study things that are well lit.
I don't know what kind of reference you're looking at for your studies, but judging by the value ranges I see in them they're probably pretty flat and low contrast. If you want to learn how to show form, it's much better to find a higher contrast picture and really see how it works, really see how values change across a shape and show its 3D-ness. If you look at old master paintings and old master studies, most of them have pretty intense, direct lighting with fairly strong shadows. There's a good reason for this: it's easy to show the forms, which was a key interest for several centuries in art history (renaissance and onwards) and it's easier to study and understand properly. Low contrast and subtle lighting is more recent, in general.
Charcoal is nice because it goes dark very quickly, it gradates really nicely and its easy to work with, if just a bit messy. The key with charcoal in my opinion is to really make use of its advantages and limitations. Use it's ability to smudge fully so you can play with a wider range of values and experiment with what you like.
If a complex form is too much to work from, find some simple things to draw. Still lives are usually pretty good. If you can't find any good b&w photographs, set yourself up a still life with some basic geo forms (cups, boxes, fruits, eggs etc) and shine a fairly strong light on it, and try to minimize ambient lighting from windows and other light sources if you can.
Some examples of good lighting setups. Not necessarily to study from, but this is the kind of contrast you're after
http://starduskmoon.deviantart.com/art/D...-112798987
http://starduskmoon.deviantart.com/art/D...-112603013
http://sanstock.deviantart.com/art/Spher...-338205339
https://s-media-cache-ak0.pinimg.com/236...e03ce5.jpg
http://webneel.com/sites/default/files/i...torial.jpg
http://academyofrealistart.com/arawp/wp-...te_300.jpg
https://mhsart1.wikispaces.com/file/view...70/geo.jpg
http://1.bp.blogspot.com/_M165yuhllZ0/TK...f-cars.jpg
It was harder than I thought to find simple form to render, but that's the idea. Distinct light and shadow side, lighting set up in a way that when you sit you see both and in good proportion and a variety of shapes and forms to work from. Studying from B&W photography will make it easier to see the values, if its a good picture, but if you cant find anything decent, nothing beats a desk lamp and some fruits. Oh and master charcoal drawings of figures are good to study as well.
Remember, the idea is to exaggerate the values so you show the form. Subtlety can come later, if you choose, but there's no replacing a good foundation in a strong lighting sense. Also remember to think about what you're looking at. Don't just copy, try to take some notes. Try to identify as many steps of value as you can.
http://1.bp.blogspot.com/_eqdCOIVZfs4/TK...w+Demo.jpg
http://georgetownatelier.com/wp-content/...milies.jpg
Hope that helps