There are the following textbooks to learn about infinite-dimensional manifolds:
"The Convenient Setting of Global Analysis" by Andreas Kriegl and Peter W. Michor
"Functions on Manifolds: Algebraic and Topological Aspects" by V. V. Sharko
"Diffeology" by Patrick Iglesias-Zemmour
"Fundamentals of Differential Geometry" by Serge Lang
"Foundations of Global Non-Linear Analysis" by Richard S. Palais
"The Inverse Function Theorem of Nash and Moser" by Richard S. Hamilton
"Manifolds, Tensor Analysis, and Applications" by Ralph Abraham, Jerrold E. Marsden and Tudor Ratiu
"Foundations of Mechanics" by Ralph Abraham and Jerrold E. Marsden
"Analysis, Manifolds and Physics" by Yvonne Choquet-Bruhat and Cecile DeWitt-Morette
"Riemannian Geometry" by Wilhelm P.A. Klingenberg
"Infinite-Dimensional Lie Groups" by Karl-Hermann Neeb
"Nonlinear Functional Analysis" by Klaus Deimling
"Real and Functional Analysis" by Serge Lang
"Differential and Riemannian Manifolds" by Serge Lang
"The Dolbeault complex in infinite dimensions, I." by László Lempert
"The Dolbeault complex in infinite dimensions, II." by László Lempert
"The Dolbeault complex in infinite dimensions, III." by László Lempert
"On the differential geometry of infinite-dimensional Lie groups and its application to the hydrodynamics of perfect fluids" by Vladimir I. Arnold
"Geometric Aspects of General Topology" by Katsuro Sakai
"Topology of Infinite-Dimensional Manifolds" by Katsuro Sakai