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 (archive.org download link)
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.

Antennas(archive.org online book link)
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.

Inductance Calculations 
By Frederick W. Grover, PHD.  -- 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 syste

Practical Electronics for Inventors
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.

The book Care and Feeding of Power Tubes has been generously made available courtesy of Communications and Power Industries (cpii.com — Eimac is a division of CPI). The book consists of six PDF sections covering all phases of tube operation and design.
Chapter 1 - Table of Contents and Figure List
Chapter 2 - Introduction and What Is a Power Grid Tube?
Chapter 3 - Electrical Design Considerations
Chapter 4 - Linear Amplifier and Single-Sideband Service
Chapter 5 - Neutralization
Chapter 6 - Operating Conditions for Various Applications

http://www.egpreston.com/ac.htm
An AC circuit analysis program, complete with source code.

http://www.zerobeat.net/G4FGQ/
Reg Edwards, G4FGQ's Software -- A collection of free programs for designing antennas.

The unofficial Numerical Electromagnetic Code (NEC) Archives
NEC Archives--these programs model antennas using the method of moments technique described in Kraus' Antennas.

http://www.eznec.com
Roy LeWallen, W7EL, has retired and made his full featured antenna modeling program, EZNECPro + v 7.0 , available as a free download..

Bell System Technical Journal, 1922-1983.  A treasure trove of research and development papers produced by what what used to be "The Phone Company."

The US Department of Agriculture has produced a guide bulletin that provides the information needed to properly design guying for conductors attached to wood distribution poles.  The bulletin contains data, equations, and sample calculations.  This may assist hams who want to guy masts.