What's with the a,b,c thing? In any case after downloading two updates in two days I'll wait till they post a stable patch since I do not have unlimited bandwidth.

Should I download this patch, or will another build come up in the next five minutes?

Big compliment to the developer(s) (and/or the GOG-Staff) for bug-fixing and stuff! :)
But I'll wait until this flood of updates will come to an end and download the big package then. Would be nice if you tell us when it's finished. ;)

Changelog for Patch 2.0.5c/ GOG-24 (Windows)/GOG-5 (Linux)/ GOG-40 (Mac) (added 1 July 2016):

Performance:
- Optimize pathfinding
- Kill memory leak
- Make sure projectiles are properly destructed & recycled
- Make sure shadows are properly destructed & recycled
- Make sure HP bars are properly destructed & recycled
- Make sure effects are properly destructed & recycled
- Make sure enemy wave blocks are properly destructed & recycled

Bugfixes:
- Fixed a crash bug on beating bonus levels
- Fixed crash bug with mods
- Crashdumper will provide a helpful notice on crashing
- Turning off crash reports will also disable crash-dumper (possibly improves performance if you disable crashdumper, not

sure. Certainly doesn't hurt performance to disable crashdumps).
- Added safer file saving (gracefully prevent crashes if file permissions forbid saving)
- Added warning if game cannot autosave
- Fix crash in mod interface
- Fixed the crashdumper so crash log files will now be produced in your game's save directory! (Screenshots on crash is currently broken, but everything else is logged)
- Fix some random crashes
- Make sure battle layouts are preserved in imported saves
- Removed redundant achievements.xml file write when boosting to level 5 (caused slight lag)

Cumulative updates today:

Patch 2.0.6b (06 July 2016)

-The Linux version will be updated as soon as possible.
-Fix crash when pressing battle hotkeys in a pop up menu

Patch 2.0.6a (06 July 2016)

-Fix crash associated with battle numbers

Patch 2.0.6 (06 July 2016)

- Fix NG+ achievement mixup with endless modes
- Fix ice mage parallel ice shard bug
- Increase ice mage orthogonalization
- Fix "idle healer" bug
- Fix E.T. health bar display bug
- Fix vs_type bug in Archshepard Stave & The Beast's Weapon
- Fix bonus progress display
- Fix "strange symbol" display in foreign locales
- Fix save slot time display
- Make "flashing effects" on by default
- Fix "Error Sheep" appearing in NG+ boss battle
- Minor cutscene fix

Patch 2.0.7a (08 July 2016)

-Show high scores in bonus level previews
-Fix scaling for original art mode in shops & you got screens
-Fix book status effects, should not apply to bosses
-Fix phantom cutscenes on beating "The Way Out" & restarting
- Crash guard in party screen on skill assignment
- Fix status effect icons in battle defender menu
- Fix Achievement Popup timing bug
- Fix Achievement location in battle
- Fix Achievement pop-under bug
- Fix range preview boost artifact on deselect
- Fix furious end book
- Fix mute button inconsistency
- Fix "double evni" upgrade/equip bug
- Fix item tooltips for defenders in battle
- Fix status effect offset in defender menu
- Fix mcguffin health bar on failure not @ 0
- Right click now unselects spell, defender, enemy, etc.
- Fix rare crash with swimming enemies
- Fix retro art equipment icon artifact in party/reward menu when nothing is selected

Patch 2.0.8b (21 July 2016)

-Added the OpenGL version of Defender's Quest (Windows) that uses OpenGL rendering instead of ANGLE. Any players who experience driver or compatibility issues with the normal windows build can try this instead.
-Fixed skill upgrade green numbers & missing stats
-Fix infinite white screen bug in ET battle
-Fix typos in tips & the journal
-Fix sneaky video button in battle via accessibility
-Fix too long names in redesign menu
-Fix missing dialogue images in ET battle
-Fix offset icons w/ bad tooltips in enemy status effect preview
-Fix bleed effect visual
-Fix post-ET puppet show
-Make ET's dark heal spell look different
-Localization:
-Add goodies/mod files
-Fix German journal
-Fix many (but not all) text overflows
-Fix some English typos
-Update Spanish text
-Lots of other things
-Fix untranslatable text files
-Fix untranslated text in many screens
-Fix problem with spell icon text in non-English locales
-Fix "book of blood" + "zap" spell insta-murder bug
-Fix untranslatable reward text in map preview
-Fix untranslated tutorial/dialogue
-Fix crash
-Hide level of certain bosses
-Fix "upgrade all weapon" achievement
-Fix stat boost special for certain items
-Show numberical value of enemy "barrier" stat (ET)

Changelog for update 2.0.8c / GOG-28 (Windows) / GOG-44 (Mac) / GOG-8 (Linux) (added 22 July 2016):

- fixed: a minor visual annoyance in the party menu (spells like lightning had stats of "0" even though they still worked properly)

Changelog for Patch 2.1.1b / GOG-29 (Windows) / GOG-9 (Linux) / GOG-45 (Mac) (added 07 December 2016):

- Added controller support
- Fixed performance bottlenecks
- Fixed various crashes
- Added soft keyboard overlay for gamepad text input
- Initial stab at Japanese/Korean soft keyboard input for those locales. (Could be totally wrong! Please provide feedback if you can)
- Fixed incorrect wave count bug in endless battles
- Fixed bug in overworld where you could click on off-screen battles
- Fixed unselect defender on select bug
- Fixed journal crash when resolution is small
- Fix glitch with some HP bars
- Fixed tileset glitch at desktop resolution
- Fixed missing lines in cutscenes
- Dropped support for 32-bit Linux, 64-bit only from now on

Changelog for Patch 2.1.2c (28 December 2016)
- Fixed overworld scrolling bug

^ plus the following notes (since this is a cumulative update).

Patch 2.1.2b (28 December 2016)

-Fixed skills/equip button toggle in party menu for gamepad
-Fixed wrong recruit screen button
-Fixed "infinite buy" bug on shop screen for gamepad
-Fixed inconsistencies with mouse cursor

Patch 2.1.2a (28 December 2016)

-Added a equip/skills toggle to party screen for gamepad & SC mode
-Added mouse-clickable rename/reset pts btns to party screen for SC mode
-Fixed soft keyboard input for SC (was broken)
-Added gamepad button to recruit/redesign screen for easy renaming
-Added cycle defenders back/forth to Left & Right grip on default SC config
-Fixed crash bug for windowed mode
-Fixed bug where fancy mouse would be forced even if you'd selected default

^ And this:

Changelog for Patch 2.1.2 (added 28 December 2016):

- Fixed lots of bugs with controller support
- Updated Russian localization (still some bugs left that are my fault, not the translators')
- Fixed various crashes
- Replaced triangular "fancy" cursor with hand-shaped "fancy" cursor
- Fixed a bug in the final battle where enemies who reached an unoccupied alternate end point would damage the main character immediately instead of turning around
- Fixed incorrect shade background in bonus menu
- Avoid insta-clicking bug in party menu with controller
- Fixed broken tutorial in party menu with controller
- Fixed invisible characters in party summon menu bug
- Fixed "phantom fireball" bug on crystal explosion
- Fixed bug where "use same party formation as last time" would allow you to summon characters on top of crystals

Changelog for 2.1.2c / GOG-46 (Mac) / GOG-10 (Linux) (added 05 January 2017):

- Mac and Linux version updated from 2.1.1b to 2.1.2c

Changes see above

The Mac version was also updated to ver. 2.1.2c (GOG-46), though there's no changelog/notification about it.

Windows, Linux, and Mac are all ver. 2.1.2c now, the changelog for which seems to be this one.

Ah, makes sense. I wasn't aware Mac and Linux version were still 2.1.1b.
I've updated the post above. Thanks for the heads-up!