In the past I've shied away from advertising because I felt it was intrusive. According to Google Analytics I average 100,000 page views per month.
Also consider setting up a mailing list which you use to periodically send out updates on interesting things you're working on.
That way onces you have something you want to promote (be it a book, website, event, etc.) you'll have a solid following to which you can promote it.
How do you get there? A/B testing of course.
Now the questions become:
A) Would you rather spend your time writing, conducting, and analyzing A/B tests or writing another book?
B) Which offers a better opportunity to scale, the website or another book?
Finally, there is nothing wrong with leaving some "meat on the bone" for your readers by not fully monetizing your site. The author reader relationship is intimate, and intimacy requires trust. Rotating ads based on where I've browsed are creepy.
If you go down the advertiser route, it should be direct placement of goods and services you are willing to put your name behind and which you believe are in the interest of your readers.
Good Luck.
Many people will probably suggest an ebook/course/etc, but they're all just slight variations on your book.
See examples: http://www.marco.org/sponsorship
I've also seen you show a few products in some posts so affiliates may be an okay money maker (See more money than google ads).
The best would be to have someone pay to sponsor you per month or per period.
Anyway, I completely agree on the avoidance of advertising. We are British after all. But advertising gets you money for almost no effort - you already have 100K visits, you do no extra work. Sadly that's about the only time you will get money for nothing. Here's my list of the alternatives.
1. The simplest and apparently best advice is to build up a mailing list. I will happily receive a monthly mail from you telling me about more London Bus projects, how the plan28 is coming along and so on. Keep it personal. Mailchimp is your friend here. Its pretty good.
2. Anyway, the point about the mailing list is they are supposed to have seriously good response rates - 20% plus (see patio11). Every so often you can monetize that list by selling ... and this is where the issue hits. You need something to sell, and that's a new book, a training course (on London bus model modding, or atmospheric balloon flying). And that something is as much work as a small book. So its not like you are monetizing the website with no effort.
3. Recruitment models seem to work well - you have access to a huge pool of technical talent, you have a reputation for integrity and projects that go somewhere. Why not put the word out on your blog that companies who want long-term hard but interesting projects to be taken on by the best freelancers should contact you.
You pick the projects, put out a call for team members and take a recruiters cut (20% first year salary).
I know that's not monetizing the blog - its taking on a new job as a project manager. But you do something similar for plan28 I guess. Anyway, think the stuff at bloomberg research (https://news.ycombinator.com/item?id=5637739) not "I want a new website".
If it works well you can hire a real PA and just do the fun code reviews.
4. Travel agency model - I listened to a really good podcast on Roman Empire a couple of years ago, (I came in about 100AD and so had 900 years of catchup before I hit the "what a whole week before I get to listen again!")
Anyway, he offered to do small group tours of Italy and Rome - it seemed pretty successful, he did not get rich but he essentially got first class tickets and holiday in Rome, doing what he loved plus a fair bit of spending money.
Why not do the same for your geek atlas? There are a number of boutique tour operators who would probably help out. And you get a free holiday.
5. I get the sneaking feeling you are "looking about" for something new to excite you (you strike me as a guy who needs a lot of projects to keep the brain tricking over and you just dropped below the threshold)
6. Labs. My own personal plan is to put enough aside consulting to pay my own tenure - and to then develop products in 'almost-emerged' industries (Computer-vision, UAV).
This is really not website stuff - but if it is new projects you are looking for and monetizing somehow, well, an evening in the pub brainstorming will come up with enough to keep you busy for a year or two.
Actually that last one is interesting - if this is still bugging you in a couple of weeks, just tell #HNLondon that you will be at the Dog and Bone after the meetup for a pipe-smoking, real-ale drinking, brainstorming for freelancers.
If you and five others do not walk away at the end of the evening with a fully planned out 18 months of work, then none of you were drinking strong enough beer.