N. Blackburn, Finite groups in which the nonnormal subgroups have nontrivial intersection, Journal of Algebra 3 (1966), 30–37.
G. A. Fernández-Alcober and L. Legarreta, Conjugacy classes of non-normal subgroups in finite nilpotent groups, Journal of Group Theory 11 (2008), 381–397.
G. A. Fernández-Alcober and L. Legarreta, Bounds for the number of conjugacy classes of non-normal subgroups in a finite p-group, Communications in Algebra 37 (2009), 1–15.
R. La Haye and A. Rhemtulla, Groups with a bounded number of conjugacy classes of non-normal subgroups, Journal of Algebra 214 (1999), 41–63.
G. A. Miller and H. C. Moreno, Non-abelian groups in which every subgroup is abelian, Transactions of the American Mathematical Society 4 (1903), 398–404.
O. J. Schmidt, Groups having only one class of nonnormal subgroups (Russian), Mat. Sbornik 33 (1926), 161–172.
O. J. Schmidt, Groups with two classes of non-invariant subgroups (Russian), Memorial volume dedicated to D. A. Grave, pp. 291–309, publisher unknown, Moscow, 1940.
V. M. Sitnikov and A. D. Ustjuzaninov, Finite groups with three classes of noninvariant subgroups (Russian), Uralskiĭ Gosudarstvenniĭ Universitet, Matematičeskie Zapiski 6 (1967), 94–102.