I remember how long it tooks to view current directory in Norton Commander when number of files >500.
With number of books > 500 it's not good way to use FAT file system and trust me data base will search for file faster then file system, just because this is data base's job.
Another aspect is a limitation of FAT32:
a) The maximum possible number of clusters on a volume using the FAT32 file system is 268,435,445 http://support.microsoft.com/kb/184006
b) Cluster size for file on FAT32 for 32GB: 16 KB, so every thumbnail will cost you no less then 16K. So 150000*4 files will cost no less then 9.6 GB on FAT32.
c) Every entry occupied 4 bytes in FAT32 table (up to 8 MB). http://www.pcguide.com/ref/hdd/file/partFAT32-c.html
P.S. A program, that I wrote for home use, a film catalog, uses sqlite for storing all data for a film, including several pictures, db file today about 60MB in size and it works as a sharm (Win32 )