resize table to fit landscape page

In general, I do not like tables to be in the text, data should be in an appendix and be referenced. Tables in text are always disturbing. In this case, as you have long data, I would not bother about table conventions and split the table lines.

documentclass[twoside,12pt]{book}
usepackage{pdflscape}
    begin{document}
    begin{landscape}
    begin{table}[ht]
defnameofcol#1{textbf{#1}}
caption{textbf{GlobalparametersoflowredshiftAGN(z$<$1.0).}}
    begin{tabular}{lllllllllll}
    nameofcol{Colname}&nameofcol{test}&nameofcol{test}&nameofcol{test}&nameofcol{test} 
    nameofcol{Nextline colname}&nameofcol{test}&nameofcol{test}&nameofcol{test}&nameofcol{test} 
    hline 
    1&2&3&4&5
10000006&7&8&9&1000000
hline 
1&2&3&4&5
10000006&7&8&9&1000000
hline 
1&2&3&4&5
10000006&7&8&9&1000000hline 
1&2&3&4&5
10000006&7&8&9&1000000hline 
end{tabular}
end{table}
end{landscape}
    end{document}
   

Using tabular*, macro extracolsep{fill}, footnotesize for font size and larger textheight (if this is not possible, then you need smaller font size):

documentclass[twoside,12pt]{book}%{Latex/   Classes/PhDthesisPSnPDF}
usepackage[vmargin=25mm]{geometry}
usepackage{pdflscape}
usepackage{booktabs, makecell}



begin{document}
begin{landscape}
begin{table}[ht]
footnotesize
    centering
    caption{textbf{Global parameters of low redshift AGN (z $<$ 1.0).} }
    label{LowRS}
setlengthtabcolsep{0pt}
    begin{tabular*}{linewidth}{@{extracolsep{fill}} cccccccccc }
        toprule
Sl No   &   Object Name &   RA  &   Declination 
        &   makecell{Redshift (ULDA)} 
        &   makecell{Redshift (NED)} &   N$_{HI}$ &   E(B-V)  
        &   makecell{No of Spectrum}  
        &   makecell{(LWP + LWR + SWP)}            
        midrule
%% table body
1   &   Mrk 335 &   0h 03m 45.10s   &   19d 55m 45.15   &   0.025   &   0.02578 $pm$   0.00006 &   3.3E+20 &   0.598911071 &   77  &   23  +   12  +   42  

2   &   NGC 1275    &   3h 16m 29.63s   &   41d 19m 51.8s   &   0.017   &   0.01756 $pm$   0.00004 &   1.35E+21    &   2.450090744 &   28  &   6   +   4   +   18  

3   &   Mrk 421 &   11h 01m 40.30s  &   38d 28m 34.0s   &   0.031   &   0.03002 $pm$   0.00008 &   1.33E+20    &   0.24137931  &   319 &   118 +   20  +   181 

4   &   NGC 3516    &   11h 03m 18.11s  &   72d 49m 00.7s   &   0.009   &   0.00884 $pm$   0.00002 &   3.08E+20    &   0.558983666 &   120 &   19  +   11  +   90  

6   &   QSO 1226+023    &   12h 26m 33.32s  &   2d 19m 43.2s    &   0.158   &   0.15834 $pm$   0.00007 &   1.68E+20    &   0.304900181 &   473 &   201 +   17  +   255 

5   &   NGC 4151    &   12h 08m 01.06s  &   39d 41m 01.8s   &   0.032   &   0.00332 $pm$   0.00001 &   2.1E+20 &   0.381125227 &   1080    &   400 +   90  +   590 

8   &   Mrk  478    &   14h 40m 04.60s  &   35d 38m 53.0s   &   0.079   &   0.07906 $pm$   0.00026 &   9.05E+19    &   0.164246824 &   49  &   14  +   12  +   23  

7   &   NGC 5548    &   14h 15m 42.14s  &   25d 22m 00.2s   &   0.017   &   0.01717 $pm$   0.00002 &   1.55E+20    &   0.281306715 &   402 &   147 +   23  +   232 

9   &   Mrk 841 &   15h 04m 01.2s   &   10d 26m 16.15s  &   0.036   &   0.03642 $pm$   0.00012 &   2.01E+20    &   0.364791289 &   28  &   10  +   1   +   17  

10  &   QSO 2155-304    &   21h 55m 58.23s  &   30d 27m 51.8s   &   0.117   &   0.116   $pm$   0.002   &   1.29E+20    &   0.234119782 &   597 &   278 +   13  +   306  
    bottomrule
    end{tabular*}

    caption{textbf{Global parameters of intermediate redshift AGN (1.0 $<$ z $<$ 3.53).}}
    label{InterRS}
setlengthtabcolsep{0pt}
    begin{tabular*}{linewidth}{@{extracolsep{fill}} cccccccccc }
        toprule
Sl No   &   Object Name &   RA  &   Declination
        &   makecell{Redshift (ULDA)}
        &   makecell{Redshift (NED)} &   N$_{HI}$ &   E(B-V)
        &   makecell{No of Spectrum}
        &   makecell{(LWP + LWR + SWP)}          
        midrule
%% table body
    1   &   QSO 215+015 &   2h 15m 13.50s   &   1d 30m 54.0s    &   1.715   &   1.715   $pm$    N/A    &   3.35E+20    &   0.607985481 &   11  &   (5  +   5   +   1)  

    2   &   PG 0946+301 &   9h 46m 46.30s   &   30d 09m 19.0s   &   1.216   &   1.22345 $pm$   0.00044 &   1.47E+20    &   0.266787659 &   11  &   (4  +   0   +   7)  

    3   &   QSO  57+561A    &   9h 57m 57.30s   &   56d 08m 22.9s   &   1.39    &   1.4134  $pm$    N/A    &   8.78E+19    &   0.159346642 &   20  &   (10 +   8   +   2)  

    4   &   PG 1241+176 &   12h 41m 41.00s  &   17d 37m 29.0s   &   1.273   &   1.273   $pm$    N/A    &   1.94E+20    &   0.352087114 &   7   &   (6  +   0   +   1)  

    5   &   QSO 1246-057    &   12h 46m 28.92s  &   -05d 42m 58.3s  &   2.212   &   2.247   $pm$   0.0016  &   2.11E+20    &   0.382940109 &   1   &   (0  +   1   +   0)  

    6   &   PG 1247+268 &   12h 47m 39.00s  &   26d 47m 28.0s   &   2.041   &   2.038   $pm$    N/A    &   9.4E+19 &   0.170598911 &   2   &   (0  +   1   +   1)  

    7   &   PG 1329+412 &   13h 29m 29.90s  &   41d 17m 23.0s   &   1.935   &   1.93866 $pm$   0.00044 &   9.1E+19 &   0.165154265 &   3   &   (0  +   2   +   1)  

    8   &   QSO1331+170 &   13h 31m 10.10s  &   17d 04m  24.0s  &   2.084   &   2.084   $pm$    N/A    &   1.59E+20    &   0.288566243 &   4   &   (3  +   0   +   1)  

    9   &   PG1338+416  &   13h 38m 52.20s  &   41d 38m 18.0s   &   1.219   &   1.21708 $pm$   0.00044 &   7.97E+19    &   0.144646098 &   4   &   (1  +   0   +   3)  

    10  &   QSO1442+101 &   14h 42m 50.55s  &   10d 11m 13.0s   &   3.53    &   3.55234 $pm$   0.00052 &   1.51E+20    &   0.274047187 &   4   &   (1  +   0   +   3)  

    11  &   PG1634+706  &   16h 34m 51.70s  &   70d 37m 37.0s   &   1.334   &   1.334   $pm$    N/A    &   5.58E+20    &   1.012704174 &   20  &   (15 +   2   +   3)  

    12  &   PG1718+481  &   17h 18m 17.70s  &   48d 07m 11.0s   &   1.084   &   1.084   $pm$    N/A    &   2E+20   &   0.362976407 &   3   &   (0  +   1   +   2)  

    13  &   QSO2126-158 &   21h 26m 26.69s  &   15d 51m 51.5s   &   3.275   &   3.268   $pm$   0.001   &   4.46E+20    &   0.809437387 &   4   &   (0  +   1   +   3)  

    14  &   QSO 2204-408    &   22h 04m 33.00s  &   40d  51m 35.0s  &   3.17    &   3.155   $pm$   0.001   &   1.08E+20    &   0.19600726  &   10  &   (3  +   0   +   7)  

    15  &   QSO2223-052 &   22h 23m 11.05s  &   5d 12m 17.0s    &   1.404   &   1.404   $pm$    N/A    &   4.88E+20    &   0.885662432 &   26  &   (19 +   3   +   4)  
        bottomrule
    end{tabular*}
    end{table}
end{landscape}
end{document}