While hams aren't expected to be engineers, it can certainly be helpful for advanced amateurs to avail themselves of engineering textbooks, references, and computer programs intended for students and professional engineers. While textbooks are relatively expensive, the education and insight they can provide may be well worth the cost. Often, old editions are perfectly suitable for hams. Similarly, demo programs available for a free download have all the functionality needed for most of the simple engineering tasks undertaken by hams. While they may not be able to fully simulate a design, they can often get close enough to allow easy optimization by trial and error.
Handbook of Filter Synthesis
By Anatol Zverev -- A wealth of charts and tables for designing LC filters without a computer--an excellent starting point for designing filters with a computer. It is now available in paperback form.
By John D. Kraus W8JK. -- While too theoretical for the average ham, this is the book for any ham who wants to better understand the theory behind antennas and how they are modeled by computers. The Second edition covers methods of moments, the fundamental concepts behind computer programs like NEC2. The first edition, published in 1950, is too early for computers. It is now on its third edition.
Art of Electronics
By Paul Horowitz and Winfield Hill -- This book is meant to help physics students gain the Electrical Engineering knowledge necessary to be successful in their careers. Well written, with only a little mathematics.
By Frederick W. Grover, PHD. (Dover Phoenix edition) -- This classic, originally published by Van Nostrand Company Inc. in 1946, attempts to allow the calculation of many types of inductors using tables. While intended for low frequency work in which the skin effect can be ignored, it is still useful as a reference or starting point.
Radio Antenna Engineering
Dave Platt AE6EO has made Edmund A. Laport's textbook Radio Antenna Engineering available in electronic form. According to Dave, Radio Antenna Engineering was published in 1952, and presents an excellent overview of the state of commercial antenna system engineering as practiced in the first half of the 20th century. As its name implies, it's not solely about electromagnetic or radio or antenna theory although these issues are certainly a part of what it talks about. Rather, it focuses on matters surrounding the nuts and bolts (and logs, beams, bars, wires, and insulators) of actually designing and implementing a large-scale antenna system
Discrete-Time Signal Processing
A.V. Oppenheim (Massachusetts Instutute of Technology) and R.W. Schafer (Hewlett-Packard Laboratories) presents a modernized approach (in 1008 pages) to electronics engineering, in particular for communications applications, that emphasizes state-of-the-art computer-aided analysis and design techniques, as applied to signal design and signal processing.
Discrete-Signal Analysis and Design
William Sabin, W0IYH presents an introduction to modern methods of discrete-time and discrete-frequency signal analysis and design, which are rapidly becoming an important approach in electronics projects. It helps experimenters, radio engineers and students to move beyond traditional continuous-signal pencil-and-paper methods.
This book by Paul Scherz and Simon Monk contains a LOT of projects, practical information, and generally useful stuff about electronics in general. Not a lot about RF, but many other subjects are covered, from circuits and components to optoelectronics and microcontrollers.
This is broad and deep text by Thomas H. Lee covering VHF and UHF technology with a very practical orientation. The text alone is worth reading ("who really invented radio?") There's good info on oscillators, filters, antennas, and hints about how to handle stripline.
An AC circuit analysis program, complete with source code.
Reg Edwards, G4FGQ's Software -- A collection of free engineering/modeling programs.
A collection of Basic programs written by George Murphy, VE3ERP.
The unofficial Numerical Electromagnetic Code (NEC) Archives
NEC Archives--these programs model antennas using the method of moments technique described in Kraus' Antennas.
A demo program of the popular EZNEC program written by Roy LeWallen, W7EL, for modeling antennas made out wire and tubing.
Information on using VOACAP, a free program from the NTIA/ITS (National Telecommunications and Information Administration/ Institute for Telecommunication Sciences)
ITS High Frequency Propagation Models
The Institute for Telecommunication Sciences Web site's page for downloading high frequency propagation models. The ITS is the research and engineering branch of the National Telecommunications and Information Administration. Description and source code for the Longley Rice Irregular Terrain model
Mini Circuits has a page for downloading application notes on RF circuits.
Bell System Technical Journal, 1922-1983. A treasure trove of research and development papers.
A theoretical evaluation of Ultra Narrow Band technology by Phil Karn KA9Q. The high signal to noise ratios required make UNB impractical in amateur radio, where we are often tasked with communicating with signals just above the noise.