What is a good book to learn about UPS systems?

uninteruptible power supplies

there is no single book that covers all the UPSs, however you can read the famous UPSs manuals to get a very good and clear pictures about theories and way to work, i recomend to read Emerson Liebert UPSs manuals as they are clear and easy to acquire from their web sites, for small single phase UPS, read the GXT manual: 

http://www.emersonnetworkpower.com/documents/en-us/products/acpower/rackmountups/documents/sl-23185.pdf

for meduim sized UPS, read the Npower manual: 

http://www.emersonnetworkpower.com/documents/en-us/products/acpower/largefacilityups/documents/sl-24534.pdf

for the big UPSs, read the S600\610 series UPS manual: 

http://www.emersonnetworkpower.com/documents/en-us/products/acpower/largefacilityups/documents/sl-25110.pdf

i would like to point out that there is alot of good information about the UPS installation available in their website in the installation manula, but i assume you are looking for theoretical information.