Yesterday I received a book I requested that had a hand written address on it and sent via first class USPS.  The postage was about 70 cents cheaper than if the send had printed the PBS lable with the standard media mail rate on it.

(the book was paperback and very very light)

So, is it okay to hand address and to send first class with cheaper shipping for lighter books? 

Yes, it is.  However, if I am sending a small book I will generally just take a sharpie and cross out media on the label so it can go first class.

Keep in mind that if you had address the label, you must put the Request ID on the upper corner of the return address.  That is handy to have should R and R need to track the book for some wierd reason.

...and be very sure that you are writing the correct info from the label. I just received one where the sender mis-read my street, but she got the zip+4 right, so I got the book. (That, and I also live in a small town & my carrier recognizes my name & that I get MANY books - LOL)

I wrap them the same and my postoffice guy just sticks a blank sticker over the media mail thing and then it goes first class.

I hand address all my books, and it works just fine.