No, it is not Sri Nathji (a.k.a. "Sri Nathaji"). Sri Nathaji has two arms, His left arm lifted high (as in lifting Govardhana Hill?). The Deity shown in the old Krsna book has four arms; the upper right holds the Sudarshana chakra, the upper left holds the Pancajanya, the conch-shell. The lower hands are lowered in the neutral pose.
And, no, it is not Sri Venkateswara (Balaji); that sublime Deity holds His lower right hand in the varada hasta (boon-giving) position, and His lower left hand is in the katyavalambita pose ("for those who surrender unto Me, the material impediments are only this high [viz., knee-high]").
