We have visited Poland, Ukraine (twice) and Santorini. Have not been to Bosnia, but have been to Slovenia and Croatia.
Not sure why you want to go to Bosnia, but if you go there, suggest you find a day to visit Dubrovnik, Croatia.
Santorini is an amazing place and we visited there on a cruise. Not sure how you plan to get to the island, but we saw the entire island in one day. The cruise ships sail into the area where a volcano blew up 3500 years ago, leaving the edges of the island, one half of the curved area is Santorini. It is my favorite Greek island, but always overrun with tourists.
Poland, is a wonderful country, we did a one week bus tour there in 1989. I know the country has changed since the collapse of the Warsaw Pact, but probably for the better, with an improved infrastructure. Krakow was the highlight of the trip, since the city was spared destruction by Marshall Zukov in WWII. Warsaw was 99 per cent destroyed. Learn the story of the Warsaw uprising in 1944 when the Red Army was at the Vistula, the Polish partisans rose up to take the city from the Germans. Stalin kept the Red Army from helping the patriots and most were wiped out by the Germans. Gdansk has a lot of history and worth a visit. Auschwitz is a terrible reminder of genocide and may give you nightmares, but I recommend a visit. The Polish people are very friendly and the food is a great surprise. The word for beer in slavic languages, including Polish is pivo.
Ukraine is still recovering from the economic collapse of the Soviet Union. You will see a difference from Poland and W. Europe. However, the people are great. Also, I agree with an earlier poster about the attractive women.
There is much history, with ancient cathedrals and monasteries as well as WWII history. Kiev is worth 3-4 days. Also, a couple of days in Odessa is recommended. Sorry that Sevastopol is now Russian, but that is an interest place.
If you haven't read the history of Ukraine, perhaps you didn't learn that during the 1930s when Stalin was forcing collectivization of the farms, 9-10 million Ukrainians were starved to death in an effort to put a stop to Ukrainian resistance to the collectivization. You will learn about that in Kiev. There are tours from Kiev to Chernobyl, if you wish. I understand that they are short. Perhaps everyone gets a radiation badge! We did not take advantage of that tour. Again, the food is good in Ukraine. I discovered some wonderful borst that I previously had not cared for. It is great in Ukraine.
Prices in Ukraine are low, and I suspect still low in Poland compared to W. Europe.
As for reading up on the history of Poland and Ukraine, you will invariably find yourself reading a lot of Russian history. I recommend a book called the Russian Chronicles. Poland has been independent at times and at times partitioned by the three empires adjoining it. Ukraine's NW half was once a part of the large Polish-Lithuanian Confederation with the Turks controlling the southern half. The Russian Empire slowly absorbed Ukraine and part of Poland. Catherine the Great was largely responsible for incorporating Ukraine into the Russian Empire. Today, most Ukrainians, especially in the eastern part of the country want to keep their independence, with more ethnic Russians in the western part of the country.
Also, there is a lot of Jewish history in Poland and Ukraine. Enjoy your trip.