I have a Japanese book by Chikako Mitsuhashi that I got from my Japanese penpal when I was in grade school. I am now 51, so it's old. So far, what my son and I were able to translate is that it is book 4 in the"A Little Love Story" series. Anyone know the value of this or where I can find out?
It sounds like you're hoping it's worth some money, but unfortunately it isn't. Manga generally doesn't gain value, even as old as that book is. This series in particular was mass-produced and the there were many re-prints. (A very rare case of manga skyrocketing in value is the first edition Sailor Moons.)
Currently any volume of Chiisana Koi no Monogatari (even the original 1970s copies) can still be bought online for about 5 U.S. dollars with a little more for shipping. Example: http://hon.gakken.jp/comic/others/?s=1&p=4 (The above was just found with a google search.)
If you ARE planning on selling it, a collector in your area may be interested, but it may take more effort to find one than it's worth. Also keep in mind that book collector's want a mint/nearly pristine copy that's been properly protected over the years; books will "turn" no matter what, but unprotected books will turn excessively so.
If you AREN'T planning on selling it and just were curious of it's value, I suggest you look up book preservation sleeves or just proper book storage. If it means a lot to you, keeping it properly stored will keep it from undue harm and from falling apart in your hands in a few years.