The (0,0,0)-(0,0,0) band of the (B) over tilde (2)Sigma(+)-(X) over tilde (2)Sigma(+) system of three isotopomers of yttrium imide ((YNH)-N-14, (YNH)-N-15, and (YND)-N-14) has been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The (B) over tilde (2)Sigma(+)(0,0,0) state of (YNH)-N-14, (YND)-N-14, and (YNH)-N-15 is severely perturbed below J=30.5 by eight, three, and two vibronic states, respectively. Although, the nature of these perturbing states can only be speculated upon, their symmetries are either (2)Sigma or (2)Pi, and this has made it possible to deperturb the (B) over tilde (2)Sigma(+) state successfully. The spectra can be reproduced within 140 MHz (0.0047 cm(-1)). The analyses confirm that the molecule is linear in both states with the nuclear arrangement Y-N-H. The bond lengths in the ground (X) over tilde (2)Sigma(+) state and the (B) over tilde (2)Sigma(+) state have been derived to be r(Y-N)=1.877 57(13) Angstrom, r(N-) (H)=1.0067(10) Angstrom, and r(Y-N)=1.8839(43) Angstrom, r(N-H)=1.242(30) Angstrom, respectively. The results are compared with the values of ab initio calculations on YNH and YN, and the experimental data on YN and YO. The atomic character of the unpaired electron in the ground state is 58% Y (+) 5s and 42% Y (+) 5p. The electron configurations for the ground (X) over tilde (2)Sigma(+) state and the (B) over tilde (2)Sigma(+) state are discussed and compared with ab initio calculations whenever possible.