This page is mostly a digest of the book "Fundamentals of Astrodynamics" by Bate, Mueller and White (known as the "BMW" after the authors). This page is useful as a reference but not really good for learning material. I find the Wikipedia pages on this subject a bit shallow too (one of these days I'll contribute improvements to the Wikipedia pages), so if you intend to learn this stuff, obtaining a book is the only viable option (if you know of other good online resources, please share with us. I don't). The 1st edition BMW book from 1971 is available for dirt cheap (15-20 USD), the 2nd edition came out just a month ago but it's poorly available (I'm still waiting for my copy from Amazon).
There's a few more pages (incl. interplanetary mission planning) if you go up one level: http://www.braeunig.us/space/
These algorithms are mostly based on the BMW book as well as a bunch of research papers from the 1960s to 1990s. Look around in different branches for some of the more advanced algorithms (including a search for closest approaches and sphere of influence transitions like Kerbal Space Program and a boundary value problem solver for interplanetary travel), they're still work in progress.
One day I'll turn this into a mission planner software for space simulator games (or some kind of game, not sure yet). If you happen to need some orbital mechanics algorithms, take a look of the above and I'm happy to answer any questions or help with this.
Spacecraft Attitude Determination and Control, edited by Wertz, is another classic. Used copies are available for around 35 USD, but new copies usually over 200 USD.
Spaceflight Dynamics by William E. Wiesel is also a good book with a broader overview into much more topics, including attitude dynamics and rocket dynamics. As opposed to the BMW which only covers orbital motion, but in more depth.
Orbital Mechanics by Prussing and Conway (of Laguerre-Conway fame) is also a very nice book, going into more details of advanced orbital motion, including some more modern methods.
But damn, aerospace engineering books are really expensive. I've spent a small fortune on these books.
Totally agree with one caveat for potential purchasers: my copy of BMW uses US/imperial units which is a little annoying if you're from a metric background. I'm not sure if there's a metric edition.
There's a few more pages (incl. interplanetary mission planning) if you go up one level: http://www.braeunig.us/space/
Finally, here's a project of mine that implements a bunch of algorithms for two body orbital mechanics: https://github.com/rikusalminen/twobody
These algorithms are mostly based on the BMW book as well as a bunch of research papers from the 1960s to 1990s. Look around in different branches for some of the more advanced algorithms (including a search for closest approaches and sphere of influence transitions like Kerbal Space Program and a boundary value problem solver for interplanetary travel), they're still work in progress.
One day I'll turn this into a mission planner software for space simulator games (or some kind of game, not sure yet). If you happen to need some orbital mechanics algorithms, take a look of the above and I'm happy to answer any questions or help with this.