Wikibooks wasn't included by mistake, it was a conscious decision to display it in the sister project snippets on the search results page and I explained why here (copied/pasted below for ease of reading).

Follow-up to the RfC for sister projects in search results
Thank you for the constructive feedback and discussion during the recent RfC. The following sister project snippets will be displayed on enwiki are listed below (in no particular order):

Wikisource
Wiktionary
Wikiquote
Wikibooks
Wikivoyage (title match only)
The following projects will not display on enwiki at this time:

Commons multimedia
Wikinews
Wikiversity
Barring any unforeseen circumstances or significant new issues being raised, we’ll enable this new feature on the search results page for all Wikipedias during the week of May 30th, 2017. Cheers, DTankersley (WMF) (talk) 10:34, 20 May 2017 (UTC)

@DTankersley (WMF): Thanks for this! This feature should really improve Wikipedia searches for the general public. I note, however, that the closure of the RfC states that there was a "strong consensus to oppose" the inclusion of Wikibooks. I disagree with the closer's assessment of "strong consensus", but I note that there was certainly a lack of support for the inclusion of Wikibooks. Would you be able to make a comment on this? — This, that and the other (talk) 03:57, 21 May 2017 (UTC)

@This, that and the other: Yes, I completely agree with you. I don't believe that there was a 'strong consensus' for not displaying Wikibooks; at the best, it got weak support. But, with only 4 people giving their opinion, I feel it's ok to display to our readers/editors/users if the content exists. I really liked @WhatamIdoing:'s response on the discussion...to note that Wikibooks is more of a site for "people seeking textbooks and related materials" and "provides non-fiction content for children" with simple concepts. The idea of displaying the sister projects is to promote discovery into new knowledge that users might not ever have seen before. Maybe we can even hope that they'll contribute and make those projects all the more better! :) DTankersley (WMF) (talk) 18:18, 23 May 2017 (UTC)

That summary at VPT was partially incorrect. From the actual RfC decision:

Wikibooks--Strong consensus to oppose.Almost every work is seemingly incomplete.A garble of poorly thought out info at best!

The consensus did not support the inclusion of Wikibooks.

Please also view the actual voting that was done on wikibooks: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(policy)/Archive_135#Wikibooks

Since the RfC results were inconclusive, I'll start a new RfC to clear up the Wikibooks issue.

New RfC posted: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(policy)#RfC:_Should_Wikibooks_pages_be_displayed_in_search_results

You developers may well devote yourselves to do your own task 'properly''--rather than trying to decode the errors of an closure--esp. without having the courtesy to ping the closure or those who were involved. Also, this may be prudential to tell you that votes don't matter, their contents matter.

@WingedBladesofGodric: Welcome to Wikimedia Phabricator. Please see the Etiquette for some remarks on constructive discussion in tasks. Thank you! :)

In T168697#3378285, @Aklapper wrote:

@WingedBladesofGodric: Welcome to Wikimedia Phabricator. Please see the Etiquette for some remarks on constructive discussion in tasks. Thank you! :)

Probably it will be more better if you folks may possess the etiquette of having the courtesy to ping the closure or those who were involved--while ''contravening'' an RFC closure purely on own thoughts.But after all, it's not unexpected!We all know a lot about the foundation!Don't we?!

@WingedBladesofGodric: Even if you may have reasons to be annoyed, there is no reason to be confrontational or artificial "you vs. me"/"them vs. us" argumentation. If you do not wish to contribute in a positive way in Phabricator, I suggest you spend your time on other topics and/or other places. Thanks for your understanding.

Last chance to weigh in at the RfC! Speak now or forever hold your peace.

@debt: The result of the RfC about including Wikibooks was "no consensus" as opinion was pretty evenly split. One idea that did have a fair bit of support was moving the Wikibooks results to the bottom, since they are probably least likely to be relevant to the user. Is that a possibility?

@debt and @kaldari The consensus did not agree to include results from Wikibooks. I still don't know why "no consensus" is interpreted as leaving everything as is. Why declining this task?

I'll give Wikibooks a few months and see.