The book series is published as two volumes, and has one of the most comprehensive treatment on Systems Analysis I've come across for the field of hydrology. It builds on Jim Dooge's treatise on 'Linear Systems Analysis'. The derivations presented here are very useful for hydrologic modeling. The book can be used to teach two semester long courses at Upper Division or Graduate level. Appendix A enlists textbooks in various subfields of hydrology and is a really good synthesis. Hathi Trust has digitized the book, so a soft copy can be accessed (though not downloaded) through university libraries that have access to it online. Would have been great to have a digital soft-copy of this book.
The only downside is that the book is a bit expensive, and requires access to Hathi Trust. Google Books which lists the holdings of Hathi Trust doesn't provide a free preview of Table of Contents, so it is hard to decide whether to purchase the book without viewing the Table of Contents.