Your primary concern should be what kind of paint is on that bookcase although research has shown that the plants we eat do not readily uptake heavy metals.
Where those heavy metals will be a problem, if any are in the soil, is on what you eat. Any more we cannot simply pull a carrot from the soil and brush it off and eat it because of the chance that some heave metal could be clinging to that carrot.

Any heavy metals in soil do not migrate very far from the source, so limiting planting near the painted sides is more effective than lining the bookcase with anything, 6 inches is usually far enough, unless you really disturb the soil. This is from the linked article, " It is important to note that plants do not absorb or accumulate substantial amounts of lead. Lead does not readily accumulate in the fruiting part of vegetables and fruit crops" and this is the link.

http://smallfarms.oregonstate.edu/sfn/su10toxicmetals

kimmq is kimmsr