Dear Deeak. Perhaps you can read the paper included in www-personal.umich.edu/~ianrit/gammaspec.pdf on the subject.

Please go to Google and enter " Books on gamma ray spectrometry". You will find several books under this with contents. From these, select the ones available in BARC library and get them loaned. You may also see whether they are available in DRP library. Alternatively, go through the latest issues of RPE journal. You will find several articles here, under each of which references to other articles and books may be available. There are also others working on gamma spectrometry in Radiation Safety Instrumentation Division of HSE group or Electronics Division of BARC.Articles will also be available from the websites mentioned under Google above.

See the attached for analysis of NaI spectra.

Thanks for response. Found https://www.osti.gov/scitech/servlets/purl/4686575 in one of the responses in attachment by Mr Alvarez and looking into it.

Dear , you can search on google about  books concerning the gamma ray spectrometry, you can find books about this field  

I glanced through the reference cited by Joseph L Alvarez. Though old, it shines as one the best extensive and educative article covering not only physics of gamma ray interactions, with reference to NaI(Tl) crystal but also methods of spectrometry with a huge compilation of decay schemes of practically all the radionuclides and analysis of their gamma ray spectrum, taken with this crystal of various sizes connected through amplifier to a multichannel analyser spectrometer. I am seeing this for the first time and it is deeply impressive. But for obtaining excellent energy resolution particularly at low energies, the latest ones using HPGe detectors are preferred. The spiky peaks at all energies obtained with this system are superior to the broadened peaks for the NaI(Tl) system. The bulky liquid nitrogen cylinders necessitated for this system to reduce noise have now been replaced by portable coolers for short term use and battery-operated spectrometers with built-in coolers are now commercially available. Papers comparing the resolution of these two systems may be accessed from nuclear instrumentation journals. The use of HPGe system for whole body counting enables measurement of low activities of gamma emitters in the body with excellent resolution. Of course, they are more expensive.

I work a lot on that problem for my PhD thesis. One of published articles is in attachment. 

It depend what you really want to measure, but usually you can just choose single gamma lines that do not interact/sum/interference with another gamma lines.

I Think that the following may give soma help

The first one, describes the problem of overlapping peak (in environmental samples) and propose a way to overcome using software methods

https://www.researchgate.net/publication/302977147_In_situ_g-ray_Spectrometry_in_the_Marine_Environment_using_Full_Spectrum_Analysis_for_natural_radionuclides

The second one is the description of the software SPECTRW that somewhere describes /show how the deconvolution of peaks is done.

https://www.researchgate.net/publication/307970414_SPECTRW_A_software_package_for_nuclear_and_atomic_spectroscopy

Article In situ γ-ray Spectrometry in the Marine Environment using F...

Article SPECTRW: A software package for nuclear and atomic spectroscopy

Dear Deepak

I do agree with answer of Sankaran Ananthanarayanan. In addition you can refer to

"Practical Gamma-ray Spectrometry 2nd Edition Gordon R. Gilmore Nuclear Training Services Ltd Warrington, UK" specifically table 16.1 ch. 16 and Appendices Tables.

Download it from google