These are the books you need to have, if you are a professional hydrologist/hydrogeologist or an engineer working on groundwater field. I also believe that when using numerical groundwater modeling, you have to understand how to solve a simplified version of the problem analytically. These books would give you much more insight into what you are trying to solve.
An afternoon at the library can save you two weeks running simulations on the computer .
Dynamics of Fluids in Porous Media (Dover Books on Physics and Chemistry) by Jacob Bear.
"A bit old, but a comprehensive treatment of the fundamentals in groundwater from a mathematical point of view. Bear has several other books (including other Dover books), which have similar content, but none are as comprehensive or methodical as this one."
Applied Mathematics in Hydrogeology by Tien-Chang Lee.
"He delivers a lot of detail that is not found easily elsewhere. For the aquifer test solutions, he goes through much of the detail associated with the integral transforms, that makes these techniques more approachable."
Groundwater by Freeze and Cherry
"A very well-written book on the basics. When you consider it was written in the 1970s, it is still largely applicable and has stood the test of time."
Aquifer Hydraulics: A Comprehensive Guide to Hydrogeologic Data Analysis by Vedat Batu.
"If you don't want to go track down all the papers associated with the classical aquifer test analysis methods, this is a good book. Some of the newer methods are obviously not in this book, but it is good cohesive introduction to the art of aquifer test analysis, though."
Analysis and Evaluation of Pumping Test Data (Ilri Pub Ser .: No 47) by Kruseman and De Ridder.
"An excellent book on aquifer test analysis. It is very hard to find a paper copy of the book, but I believe a pdf of it is available from the author on the web."
Groundwater and Wells (3rd ed.) by Driscoll.
"An ok book, but one of the few that actually discusses how to build, develop, maintain and drill water wells. It is quite slanted towards wire-wrap well screens, because it was written by Johnson well screen. I think it is too long (~4" thick!) for how much content it actually has."
Handbook of Ground Water Development by Roscoe Moss Company.
"Similar to Driscoll's Groundwater and Wells book, except that it is better written, more mathematical, and not so slanted to wire-wrap screens. Hard to find, unfortunately."
Physical and Chemical Hydrogeology by Domenico and Schwartz.
"I don't have the latest edition of this book, but the 2nd ed I have is quite good. It goes into some detail on topics not generally covered in an introductory hydrogeology book (heat convection, geology, chemistry)."
Applied Contaminant Transport Modeling by Zheng, Gordon, and Bennett.
"A well-written, practical, and comprehensive book on contaminant transport modeling, written by the author of MT3D. Some good case-studies are included as well."